Common mistakes when using the metric system | Hacker News Reader

Common mistakes when using the metric system

nayuki.io

44 pointsstereoabuse98 comments

I was surprised to see "L" as the "only correct" (though as minor mistake) abbreviation for a liter, being certain "l" or "ℓ" is the way to go, as that's how I was taught so far. Turns out that SI actually allows both "l" and "L", but I would only ever use the uppercase if it's a sans-serif font in a non-prefixed unit ("L" as opposed to "ml"). I've never seen any can or bottle using "mL" or "cL".

Some of the examples go pretty far too, either calling spelling I'd never see a "common mistake" (like "gr." for grams), or by being too scientifically correct for a casual usage, for example in a press article ("(100 + 200) grams", right).

But then I saw "Set the oven to 450 K", and I realized that the examples are to be taken with a grain of salt :)

> either calling spelling I'd never see a "common mistake" (like "gr." for grams)

Almost all cooking books I have do this (sample size is not big though, and with almost all I mean all except one). I don't know why though, maybe it's an old abbreviation which stuck in certain text types?

I've got a serious bugbear with recipes using "cups" as a measure - it infuriates me. Who in their right mind uses a volume measurement for foods that can vary hugely in their size according to how long they've been sitting around?

The vast majority of home cooking in the US measures in cups. Certainly there are people in-the-know who are going to be more precise and measure by weight, and most restaurant kitchens will probably do so as well.

I agree it's not great, but ultimately I've found it doesn't really matter in practice, even for baking most of the time. For regular cooking I do get frustrated when I'm asked to include "2 cups of shredded chicken" or the like, but ultimately it really doesn't matter. Being off won't ruin the recipe, and often more or less of a particular ingredient is a matter of personal taste anyway.

I have a kitchen scale, but it's always simpler and easier to grab and use a measuring cup, so that's what I do most of the time, unless the recipe actually calls for weight without giving volume.

I just don't get it - surely homes in the U.S. also have measuring scales? Outside of the U.S., we just go for measuring ingredients by weight which completely gets around the issue of settled ingredients or the inconsistency of measuring things by volume. Why can't recipes just use metric measurements (I'll allow the substitution of grammes for weight rather than newtons) and also provide the quick'n'easy and inaccurate cups measures too?

I live in metric land (Sweden), and I have never seen "gr." for grams and honestly I would probably take a while to understand what it meant.

yea, reads like "grains"

People do not know much about old cartridge powder measures though :D

"Grains" are still used for measuring out powder/chemicals (also arrows). If I saw "gr", I'd interpret that as grains rather than grammes as "g" is definitely grammes, so the extra "r" must signify something.

Air rifle bullets are typically specified in grains, for example

I've seen gr for grams. Not as common as just g, but it does get used every now and then.

Here in Italy is more common than any other form, I'd say

In ex-Yugoslavia countries using gr (usually without a dot) for grams used to be a fairly common thing. Last few decades as EU standardization takes place it's a lot rarer to see. Also in Croatia in everyday life people will far more often use decagrams (10g), shorten colloquially as 'deka', than grams - which leads to even more confusion because the SI abbreviation for decagrams is dag, not dg (which is decigram, 0.1g), and it's often mixed - even some primary school books had these typos.

On the capital L for litre, that’s a suggested rule (second table), not mandatory.

I also agree though, there are things there which are decidedly not mistakes (e.g. “mcg”, which is most commonly used to avoid confusion between a sloppily written mu and a sloppily written m. I’ve seen this most commonly in handwriting by doctors and vets)

gr. is not uncommon in france. Not in formal publications, but in day to day life you may find it. I know that I use it sometimes

Also avoid confusion with gravity acceleration :D

> calling spelling I'd never see a "common mistake" (like "gr." for grams)

Maybe I should change the example for gram to "gm" (or "GM"), because I actually see that in Canadian supermarkets - on the price label on shelves, not on the product label.

> I saw "Set the oven to 450 K", and I realized that the examples are to be taken with a grain of salt

I mean, that example exists to reinforce why we should continue using degrees Celsius for everyday temperatures. I'm simultaneously describing what we should do and what we should not do. The statement is less impactful if I only describe the positive example without the negative.

Lots of these are country dependent in terms of what is familiar to people. To some degree that matters if your primary communication is not international but domestic.

For e.g. in the UK sqm is very common, particularly when looking at property. On RightMove which is the most popular property website here, for e.g. you can see here that the default is `sq ft` with `sq m` given underneath: https://www.rightmove.co.uk/properties/140100683#/?channel=C...

In terms of things like mixture of units, I appreciate you want to give in one unit but I think the things chosen are a bit strange. I've never seen anyone write '15070 grams', it'd almost always be given as 1.507 kilograms.

> I think the things chosen are a bit strange. I've never seen anyone write '15070 grams', it'd almost always be given as 1.507 kilograms.

I'm sure it's just a typo, but 15,000 grams is 15 kilograms. So: 15.07 kg.

I don't think they arrived at some of the more extreme examples by accident, but I could only speculate on the intent behind them.

Definitely a typo :D I'd not had my morning coffee!

> 1 KB = 1000 bytes

But standard is kB, from kilo prefix.

Here it is either KB or KiB, depending on the meaning. IRC, KB is the only exception from the rule.

The SI standard is international and doesn't admit a K prefix, only k or Ki.

Right, the uppercase conventionally means 1024 so it should be kB or ko for 1000. There are some odd rules, like L for liter because of readability or some other reason. I've also learned in (French) school that we shouldn't use kL, that it doesn't exist. It does exist, but we apparently don't want to use it and use hL instead. Some people thing it's a linear unit and not a cubic unit. We count liters like we count potatoes and we use m3 for volume math.

Kelvin Bytes?

I've never seen Mtr, sqm or gr. Even as a child, I knew the short of those unit names.

A gr (grain) is an antiquated measurement apothecaries used

Maybe they mean "when using the metric system as an American"...

sqm instead of m^2 is also just a way of typing it on a mobile keyboard.

I’m more used to seeing “m2” on mobile, but it’s probably different between regions.

More people seem to find the ² on mobile keyboards than they do on desktop keyboards. It's still rather uncommon, though.

If anything, I would've expected phone keyboards to turn m2 into m².

I have seen usage of kvm (kvadratmeter) in Sweden

Like we need another overload of "kvm"

(Kernel Virtual Machine in Linux or Keyboard Video Monitor in server switching kit)

It is highly language dependent.

In Germany you will sometimes see Qm for "Quadratmeter" (m²).

Mostly in lowercase, especially when talking about apartments or houses

Same in Italy, mq.

Doesn't seem right. I wonder why they claim cc and Å are 'deprecated'.

cc maybe in favor of unicode's ㎤

> A cubic centimetre (or cubic centimeter in US English) (SI unit symbol: cm³; non-SI abbreviations: cc and ccm)

-- https://en.wikipedia.org/wiki/Cubic_centimetre

> Although still widely used in physics and chemistry, the angstrom is not officially a part of the International System of Units (SI). Up to 2019, it was listed as a compatible unit by both the International Bureau of Weights and Measures (BIPM) and the US National Institute of Standards and Technology (NIST). However, it is not mentioned in the 9th edition of the official SI standard, the "BIPM Brochure" (2019)[13] or in the NIST version of the same,[14] and BIPM officially discourages its use. The angstrom is also not included in the European Union's catalogue of units of measure that may be used within its internal market.

-- https://en.wikipedia.org/wiki/Angstrom#Current_status

It would have been nice to know the risks of making each of these mistakes, since honestly I only found a few places where a mistake can have real negative consequences.

As with all things, context is king. I'm not going to be confused when my colleague asks me on slack if it's OK to email me a 15mb (millibar) document - I know what they meant. Nor am I going to worry if it's actually MiB vs MB, in this context it does not matter.

Also, although it probably be nice that we all standardised on the correct prefixes, literally nobody speaks in terms of gigameters or teragrams or whatever. We carry with us an internalisation of real world measurements, and use that to compute relativity when we read/hear these numbers. It's roughly 1000km from my home town to the coast, so if if the moon is roughly 400,000 km away, then that's 400x the distance, pretty far. My car weighs roughly 1 ton, so Hafthor's 501kg deadlift record is half my car - pretty impressive!

> nice to know the risks of making each of these mistakes ... only found a few places where a mistake can have real negative consequences ... context is king

That's an excellent question. As an analogy, I would say that it's similar (but not identical) to making mistakes in natural-language spelling and grammar.

For many cases, you are correct that the meaning can be repaired with context. dis iz da kase 2 wif nglish, as u kan sea w/ this sntnce.

Likewise, there will be some cases where a single letter or word can make all the difference. "He hit her" conveys a different meaning than "He hit on her"; an ESL student might only learn the verb and not the important subsequent preposition. And then there are pronunciations: https://www.youtube.com/watch?v=3Q44_A4NzjI .

Finally, by excusing the writer of all their mistakes, it puts the onus on the reader to grasp the right meaning. And if causes the reader to be confused or misunderstand, the reader has to go back and ask questions. That's a selfish shifting of effort.

> email me a 15mb (millibar) document

It looks like the symbol for bar (unit) is "bar", not "b". It is not part of SI. https://en.wikipedia.org/wiki/Bar_(unit)

You might not appreciate this, but there are fields where mb meaning millibit actually matters. Look at information theory such as data compression and error correction codes. We might say that JPEG encodes each RGB pixel with 3.0 bits, and WebP uses 2.8 bits/pixel, so WebP is 200 millibits per pixel more efficient than JPEG. There are many codec competitions where improvements are in the range of millibits per symbol (though they're written rather normally as 0.123 bits to not scare people).

By "autocorrecting" "mb" to "megabyte(s)", we lose out on the ability to express "millibit(s)".

> literally nobody speaks in terms of gigameters or teragrams or whatever

So why do we use megabytes, gigahertz, megapascals (for some industrial chemical processes), megawatts (power plants)? It's just a matter of habit. We collectively decided that kilometre is the biggest unit we'll tolerate, even though there is nothing technically wrong with megametre, gigametre, etc. We have the same problem with avoiding megagrams, kilolitres, kiloseconds, etc.

> It's roughly 1000km from my home town to the coast, so if if the moon is roughly 400,000 km away, then that's 400x the distance, pretty far.

That's nice, and it is true that when drawing attention to a comparison, the same unit should be used (Pat Naughtin and The Metric Maven both advocate for this idea). But it's also okay to write that the moon is 400 Mm away in the context of astronomy, because very few distances between bodies in space are less than 1 Mm.

The big problem with how writers treat the kilometre is that we always end up with things like "the nearest star is 4.35 light-years or 40 trillion km away". Instead of using SI prefixes as designed, we end up jamming big number words in front of the unit, and that doesn't make things any clearer. Would you like it if I said my CPU is 3.5 billion hertz, and ban all the mega-, giga-, etc.? I could even make the case that this isn't theoretical, because for example my monitor runs at 60 Hz, and we certainly must not call it 0.000 000 06 GHz with any sense of normalcy.

> My car weighs roughly 1 ton, so Hafthor's 501kg deadlift record is half my car

And you contradicted yourself because now you're using different units. It would be better to say that your car is 1000 kg and the deadlift record is 501 kg. So it's like sometimes you accept changing units, sometimes you don't.

The most common error I see is "kph" which doesn't even have any possible meaning. 'kilos per hour' would in most places be taken as "kilograms per hour". As in "my weight increases by 3 kph when I visit McDonalds"

kph drives me up the walls.

I find all of this really funny.

kph doesn't bother me, but I'm American, and we're pretty sad and pathetic when it comes to metric, and it's similar enough to how "mph" is used here that it immediately makes sense.

What makes your specific comment funny is that we're talking about different usage of things in different languages and countries, and in one sentence you managed to express frustration at how other people say things incorrectly, while using an English idiom incorrectly. It's "drives me up the wall" (singular "wall").

I only see this from english speaking people who are used to mph

Not defending 'kph' exactly, but I can see how some native metric speakers (in Australia at least) might arrive at that solution on their own. You're right about 'kilos per hour': 'kilos' will always be taken to mean kg. But in speech, 'kays' is a common abbreviation for both km and km/h (confusingly). So 'kph' would be an ambiguous way of writing 'kays per hour'. I have heard a small number of people say K.P.H. as an initialism.

kays (distance) e.g. "Thongs'll do, we're only walking a few kays down the road."

kays (speed) e.g. "Apparently he was doing 120 kays in the Barina. I'll miss him."

kph seems to come from people that normally work in miles but switched to metric for one reason or another. The common notation I almost always see for kilometres per hour would be km/h, in rare occasions with `h` exchange for whatever letter the local translation of "hour" starts with.

I don't think anyone would confuse kph for kilograms per hour (after all, that would be kgph or kg/h, and I don't think I've ever had to calculate kilograms over time outside high school anyway). Usually, context clues make it pretty clear what's being described.

There's no confusion, the problem is simply that seeing "kph" automatically expands to "kilos per hour" in non- native English brains. That's overridden by logic, obviously, but that jarring effect is always there.

> Use 1008 MJ monthly electrical energy consumption instead of 280 kW⋅h monthly electrical energy consumption

> Avoid common non-SI units (suggested)... For serious science and engineering, SI units like metres per second should be used instead.

Yeah good luck with that... electrical power engineering speaks in kWh not MJ

> 150 pJ gamma ray instead of 938 MeV gamma ray

Particle physicists will hate you if you take electron volts from them

While I'm at it, do you know that attaching suffix to SI unit is not allowed by SI / IEC /ISO? So abbreviating megawatt of thermal energy as MWth or decibel referred to 1 milliwatt of power as dBm is technically illegal:

> "When one gives the value of a quantity, it is incorrect to attach letters or other symbols to the unit in order to provide information about the quantity or its conditions of measurement. Instead, the letters or other symbols should be attached to the quantity."

https://physics.nist.gov/cuu/pdf/sp811.pdf

Yeah... all of my spectrum analyzers, signal generators, software-defined radios, antennas and RF interface control documentation is not ISO/IEC compliant...

> good luck with that... electrical power engineering speaks in kWh not MJ

Then it's industry-specific jargon. Note that some places bill your natural gas consumption in megajoules. Food energy is quoted in kilojoules in SI, or calorie/Calorie/kilocalorie otherwise.

The problem is that if you want to make any comparisons, you need to use the same units.

An athlete eats 100 Calories, gets on an exercise bike, and powers a 100-watt light bulb for 3000 seconds. What is her thermodynamic efficiency?

To make the 500 km journey, the old car consumed 1 GJ worth of heat energy in gasoline. The new electric car consumed 300 kW⋅h. Which one consumed less energy?

> Particle physicists will hate you if you take electron volts from them

I can understand why they love their traditional unit of measurement.

The problem is that their unit measures the same type of quantity (energy) as an existing SI unit - the joule.

We've seen this play out in countless industries already, where they have their own specific units and refuse to interoperate.

Heating in BTUs, cooling in tons, explosions in kilotons of TNT, barrels of oil.

Astronomy in light-years or parsecs or astronomical units, typesetting in points, football in yards (instead of feet), microscopic things constantly compared to the width of a human hair, ~5 different scales of shoe sizes across the world.

> it is incorrect to attach letters or other symbols to the unit in order to provide information about the quantity or its conditions of measurement

I agree with this rule. I guess if you write it out in words, it would be:

* Incorrect: "Power: 480 volt-amps-reactive (VAR)"

* Correct: "Power (reactive): 480 volt-amps (V⋅A)"

* Correct: "Reactive power: 480 volt-amps (V⋅A)"

By the way, you can link to a specific page like this: https://physics.nist.gov/cuu/pdf/sp811.pdf#page=16

few notes:

- "Fuel consumption of 9.4 L / 100 km" is not "the right form" is a modern take, a more classic and still valid SI one is km/l which is very similar to "Fuel consumption of 9.4 LPK";

- "25 {kW/h,kW⋅h} to boil a tank of water" are both strange forms, kWh is the common accepted way to express energy, no center dot multiplication needed;

- multiple prefix (vs power-of-10) and bare prefix are as well commonly accepted, I see no reasons to consider them wrong, while I can state formally wrong "2 kilograms of rice" because we do not measure the mass but the weight so it should be 1.961daN where deca-newtons are commonly used because 1 daN is roughly 1kgf commonly shortened to 1kg as we can commoly count 1kg[f] == 1 daN... For instance climbing equipment in the EU use daN to express maximum loads of connectors, ropes etc because of that;

- multiple quantities much depend on industry and conciseness, as we do not write units in table values but only in headers we tend not to write them three times in a row where from the context is clear what numbers means.

The biggest issue is makes habits changes. Actually we should not use km/h as well, since for SI base unites are m/s, but 3.6 is not an easy conversion like kgf/daN, so in the EU we keep using km/h, something meaningful in the past, when we go by horses and feet, but not much needed today.

Not to count software, where often recognize "°C" (two chars) BUT not ℃ (U+2103) and so on.

> "Fuel consumption of 9.4 L / 100 km" is not "the right form" is a modern take

No, it is the right form.

> a more classic and still valid SI one is km/l

The SI one has always been L/km. Any use of km/L is a translation over from mpg (miles per gallon).

> km/l which is very similar to "Fuel consumption of 9.4 LPK"

km/l is the reciprocal of LPK. These are not remotely the same unit.

> kWh is the common accepted way to express energy, no center dot multiplication needed

In SI, it is unacceptable to multiply units together without a space, dot, or cross. This can create ambiguities - for example, "mm" could mean "millimetre" or "metre × metre". "ms" could mean "millisecond" or "metre × second".

I had a discussion here a while ago where I changed from "kWh" to "kW⋅h": https://www.reddit.com/r/Metric/comments/1313rhp/how_to_resp...

> multiple prefix ... well commonly accepted

Not really. Other than saying "I drove 30 k km", it is rare to stack prefixes. We don't say that CPU features are "5 millimicrometres".

> bare prefix are as well commonly accepted

And this is wrong. So, I have this electric scooter that goes 80 k at 30 k and weighs 40 k. How do you like that?

> we do not measure the mass but the weight so it should be 1.961daN

Actually, it's possible to measure mass. If the scale has an internal reference with a known mass, then it can adjust to the local gravitational constant.

What I mean is, let's say an electronic scale has an internal 10 g mass. When switching on the scale, it measures the internal mass and sees 9.8123 mN (millinewtons). Now it will use that scaling factor for anything else measured during the session. Then you move the scale to the south pole, and it measures the internal mass to have a weight of 9.9765 mN, so it compensates correspondingly.

Likewise, if you use a balance scale, then you are comparing against known masses (not weights). (We'll ignore buoyancy for the vast majority of commercial applications.)

> For instance climbing equipment in the EU use daN to express maximum loads of connectors, ropes etc

I haven't been exposed to that before, so thanks for that. That's just about the only example I have ever heard of involving the deca- prefix. I would much prefer all the non-power-of-1000 prefixes to die (goodbye to centi-, deci-, deca-, hecto-) for many reasons.

> we should not use km/h as well, since for SI base unites are m/s, but 3.6 is not an easy conversion

Correct. I have used both km/h (because obviously society forces it) and m/s in various applications and calculations. Using m/s is much more helpful for visualizing distances traveled in a short span of time, and also calculating acceleration (in m/s^2, never km/h/s), kinetic energy, etc.

I really appreciate this kind of plain and simple writeup, without getting into any "falsehoods programmers believe about metric"-type clickbait. Nice one.

> 81 uL of catalyst -> 81 μL of catalyst

Oh, interesting. NIST recommends "L": https://www.nist.gov/pml/special-publication-811/nist-guide-...

That line refers to the 'u' substituting for 'μ', the greek letter mu, not the L symbol. https://www.nayuki.io/page/common-mistakes-when-using-the-me...

I've always thought u for mu was a neat hack - its inconvenient to type mu and u is ascii compliant.

It usually doesn't take toooo long to figure out what u means from the context

I agree with u being a neat hack, don't agree with µ being hard to type.

On US-international keyboards, it's AltGr+m

Note: The official SI unit “liter” was written “l” until a couple of decades ago, when they changed it to “L” due to frequent legibility issues (l/I/1). Some old texts use the old convention.

they did not change it, they added it as an acceptable form

Changed? I never have seen the captial L for liter anywhere from daily use to scientific papers. It is always used in small caps.

After I read your comment and went into the kitchen and the first thing I saw was a handsoap bottle what had "1 L" written on it. Weirdly the milk had "1 ltr" on it :S

I don’t know which field you work in, but I do research in physics and chemistry and L is the dominant convention in the papers I read. I also teach intro physics at the university level and the textbook uses L.

In high school (~2008) we were taught that this was the correct new abbreviation for liter in chemistry, but I remember that at the time lower-case l (or sometimes \ell) was still normal as well. These days, I rarely see that.

(Off-topic: “Small caps” is not the same as “lower case”.)

So, when using the kelvin temperature scale, it's lower case when using "kelvin", but uppercase when using the abbreviation "K"? That annoys me as kelvin is a name and even my phone wants to capitalise it.

Same for "2.4 GHz wireless network" but it's supposed to be 2.4 gigahertz if fully spelled. I suppose there is a logic behind but it's not obvious.

60 watts , later 20 kW.

I don't know, but it seems to me that if they wanted to be scientific they wouldn't be seemingly arbitrary in the capitalization rules

I had structural engineering and electronics in school and especially when you get compound units like Nm (newton meters) not wondering if the N is a newton or the SI-prefix nano is not nothing.

In the end the reason for the weird capitalization is just to avoid situations where that ambiguity will occur.

I remember me and a friend asking our physics teacher if it wasn't a problem that tera (T) and tesla (T) were the same symbol. He looked at us, paused a bit, then explained how absolutely enormous a terra tesla would be, and thus why that isn't actually an issue :P

I've always assumed "giga" is capitalised because it one might confuse a lower case g for "grams". Technically, 1gm is one gram-meter (grams * metres), whereas 1Gm is a thousand kilometres. You don't often encounter weird combined units like gram-metres outside of physics, except maybe for kWh (kW * 1 hour), but I don't see why you wouldn't be nice to physicists and give them the extra clarity.

Other order of magnitude indicators are capitalised to distinguish them from their smaller counterparts (millimetre/Megametre).

Wikipedia states that the distinction is because units named after a person always start with a capital letter. I've always assumed Hz has to be capitalised to prevent confusion with the hecto prefix (hHz being 100Hz). I don't think there's a unit or order of magnitude that's abbreviated to `z`, so in theory "Hz" could just be "H",

I think Hz is more readable, but that's probably because I'm used to it.

> assumed "giga" is capitalised because it one might confuse a lower case g for "grams"

Note that m is milli and m is metre; overlap hasn't stopped collisions between prefixes and units.

> "Hz" could just be "H"

No, H is henry (for inductance). https://en.wikipedia.org/wiki/Henry_(unit)

I disagree with that rule. I get that people spell liter, gram, meter in lower case, but units based on names of historic persons like Volt, Ampere, Coulomb, Pascal and Kelvin deserve to be capitalized.

The unit symbols for those units (A, C, P and K) are capitalized, but the units when spelled out (ampere, coulomb, pascal and kelvin) are not, just like the spelled out forms of liter, gram and meter.

Isn't it more logical that Volt was the person and volt is the unit named after the person? To me that makes perfect sense.

No they don't. When you're actually referring to those people, then yes, you would capitalize them. When you use units named after them, you don't.

> deserve to be capitalized.

Why is that?

Capitalisation isn't universal. Languages like German capitalise every noun, so from a German perspective, Gram, Metre, and Litre should all be capitalised and it's the English that are wrong for using lowercase units.

Meanwhile, English capitalises stuff that other languages don't, like the names of languages, times (days and such), and for some reason "I". By the time the metric system was conceived, English still capitalised most nouns (just look at the US constitution for example) after very recently coming into contact with the concept of capital letters in the first place, something copied from German printing presses.

And then there are the many languages that don't even have capitals, which will probably question why there are two ways to write every letter when they first learn languages with a Latin alphabet.

Nothing "deserves" to be capitalised, every language just decided to stick to some arbitrary rules. There's no good reason why `Monday` is more important than `website`, or why `I` is more important than `you`.

The Kelvin(uppercase) scale is named after Lord Kelvin(uppercase), the unit is the kelvin(lowercase), and the symbol is K(uppercase).

This agrees with all SI units named after a person. The unit's name is lowercase (unless at the beginning of a sentence), and the symbol has its first letter in uppercase. Examples: newton (N), tesla (T), hertz (Hz), pascal (Pa), joule (J), watt (W), volt (V), ampere (A).

Note that "degrees Celsius" is capitalized in a special way.

See also: "I bought one bitcoin (1 BTC) on the Bitcoin network."

A IB Diploma Physics textbook I had used a negative one exponent instead of a division symbol in units. This is not normal right?

Yeah that's the normal way in physics in my experience, certainly from undergrad level up.

perfectly normal in physics books, maybe not usual in high school, but uni-level books all do this.

After taking Dimensional Analysis in undergrad I never stopped doing that, quite possibly to the nuisance of everyone around me...

Negative exponents do look odd. I first saw it in RollerCoaster Tycoon like 25 years ago.

Units with negative exponents are non-existent in everyday discourse (articles for the general public, product labels, commercial catalogs, etc.) and even the vast majority of engineering.

It's when you get into science and analysis where you start seeing big compound units and negative exponents. Here are a few examples: https://en.wikipedia.org/wiki/Viscosity#Units , https://en.wikipedia.org/wiki/Gravitational_constant , https://en.wikipedia.org/wiki/Vacuum_permittivity , https://en.wikipedia.org/wiki/Dimensional_analysis

I love that they included a note on the pronunciation of kilometer. It drives me nuts how everyone says it wrong.

Except they're not. In English, at least, both pronunciations are valid. I do prefer the one the article recommends, but either is fine.

This list strikes me as overly nitpicky. Certainly some of the bad example usage is a problem (e.g. mb vs. MB means two very different things, millibit vs. megabyte), but so much of this in practice just really doesn't matter.

Sure, if I'm doing a bit of formal writing, I'm going to do things correctly. But I don't see it as a problem for people to use odd abbreviations like "mtrs" or incorrectly pluralize as "kms" in informal writing. People will not have trouble knowing what you mean. No one is going to say things like "I drove 3 megameters over the past couple weeks". (Reading some of the longer explanations below seems to indicate that the author is not just talking about formal writing.)

And even in formal writing, a few of these are bit much, like good luck getting people to stop using electron-volts in favor of picojoules.

This just seems silly; it's hard to take this piece seriously, and all of the nitpicky arguments seriously undermine the (few) good arguments listed.

I thought this looked like good advice until I got to:

> Mass/weight distinction

> Incorrect: He weighs 70 kg; Correct: His mass is 70 kg

> Incorrect: She weighs 50 kg; Correct: She weighs 490 N on Earth

Really?! We’re not supposed to use kilograms to describe a person’s weight? And to describe someone’s weight, we have to use newtons and specify what planet they’re on? Good luck enforcing those rules.

Yeah, this is the only one that seems flat out wrong. When I'm on a bridge, I absolutely want to see "The bridge can hold up 3000 kg", NOT "The bridge can hold up 29 kN". Partly because "holding up" (as opposed to, say, withstanding) force sounds nonsensical, but primarily because knowing the maximum load in kg is much more useful.

You never know! Tomorrow that bridge might be on the moon, and how on earth (or how on the moon, indeed) will you know if it will hold up when you walk your herd of elephants over it!

I mean, that's still fine? The max load of that bridge on the moon will just be a different, higher value in kilograms. Since we don't move bridges between planets, it's fine to not advertise a universal value that applies everywhere.

(I get that you're joking, but still...)

Structures like bridges are a complicated case because you can't describe its load capacity as a mass or a weight (force).

I'll assume that a bridge fails when the downward force on its deck exceeds a certain constant number. But that force is inclusive of the dead weight of the bridge.

Let's say a bridge has a mass of 1000 kg and it can hold up an additional 1000 kg of vehicles on Earth. That means it can hold up about 20 000 N of weight (assuming that g = 10 m/s^2).

The Moon's surface gravity is 1/6th of the Earth's. 20 000 N of weight would be 12 000 kg of mass on the moon. Subtract the bridge's mass of 1000 kg, and you can put 11 000 of vehicles on the bridge - which is substantially more than 6× the mass of vehicles allowed on Earth.

It's all pedantry. Physics makes a distinction between mass, a (mostly) invariant property of an object, and weight, the force proportional to your mass and the planet you're standing on divided by the square of the radius. Only physicists care about this distinction.

I would like to believe engineers care too. A simple action like standing up or being on a bus that's taking a curve will make items on your person weigh more.

There's a 5% range when converting mass into weight (depends on where on Earth you are). Santa loses a few kilos when he comes down to the equator. It's not enough for people to care, but it can be important if you're building something that needs to carry a lot of weight.

This is a very important distinction to make in scientific papers. Completely meaningless for most writing. If anything, physicists got the definition for the word "weight" wrong and patched up their mistake by adding the concept of "mass". Most mass is measured standing still on earth, so "weight" is fine unless you need to be precise.

> It's not enough for people to care, but it can be important if you're building something that needs to carry a lot of weight.

The problem is that the article presents these "corrections" without being clear what context they're important in. The examples given make it sound like the author is railing against people who use these forms in regular conversational speech. And even the longer explanations after the table still make it seem like the author isn't only concerned with formal/scientific writing.

So yes, in scientific papers, I do think this list is a reasonably good guide (though good luck getting particle physicists to give up electron-volts, or people writing about electrical power systems to give up kWh). But for everyday use it feels unnecessarily nitpicky.

For the specific mass vs. weight example, especially, regular conversation (spoken or written) is pretty much always going to use the term "weight", and using "mass" would feel stiff and unnatural.

The issue is that it's very common for people to use weight when they're referring to mass. We get away with it as our weight doesn't change very much when on Earth, but if we start populating other planets, then we're going to have to become more precise.

> 6 tons of wheat 6000 kg of wheat

Why not 6 Mg of wheat?

Because clearly nobody would understand it.

Because 6 magnesium of wheat doesn't make any sense

6 Mg is technically correct and it has the advantage that 1 tonne (or metric ton) = 1 Mg exactly, so it's just a simple substitution of words without changing numbers.

At that point in the article, I wanted to ease the reader into the metric system and not introduce too many new concepts all at once.

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