Comment by jlongman | Hacker News Reader

Comment by jlongman | Hacker News Reader

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That’s funny because when I work with Fahrenheit I just work with 5°F ranges to compensate for the approximate mental math required. Eg very quick mentally, 100°F = 37.778 °C (thanks autocorrect) = (100-32)/2 = 34.

But if it was closer to freezing say 42°F =5.556 °C (again) so 5°C. So arbitrarily we could say 57°F was 12°C =53.6 °F actually.

But a true Canuck knows knowing the temp is barely half the battle, what’s the wind speed and humidity? 29°C can be a lovely day if it’s dry or completely unbearable if it’s humid.

We hardly ever use decimals for weather-related measurements, the other factors above being more relevant.

Contrast that with measurements where I would say if you need to know a precise one you should be using decimal; ie what do you do if it doesn’t precisely third or fourth? If you’re talking about tool sizes then any system works as long as your froodle matches the grommlet.

(F - 30) / 2 = ~C

C * 2 + 30 = ~F

No need to simplify this.

(F - 32) / 1.8 = C

C * 1.8 + 32 = F

I personally find the math just as easy to do accurately. For example, 87F -32/1.8 = 55/1.8 =~30.5C. Compare that to your approximate method, which would give 28.5C, which is just wrong

(Maybe I just got really good at this when working a public facing job with a lot of American tourists - they would ask what our celsius temperatures were "in real units", so I got quite comfortable converting the air and water temps. Fahrenheit never once became intuitive to me, though.)

For C to F you can often simplify the mental math by doing the multiply by doubling then taking off 10%.

E.g., to convert 31℃ to ℉: 31 x 2 = 62. Subtract 6.2 = 55.8. Add 32 = 87.8℉.

If you want to round the result to the nearest integer the subtract 10% step is a convenient place: 31 x 2 = 62. Subtract 6 (rounded 6.2) = 56. Add 32 = 88℉.

Yes, doubling then subtracting 10% of the resultant works because that is the same as multiplying by 1.8 :)

When going the other way and dividing, I similarly find it mentally easier to multiply by 10/18 (rather than just divide by 1.8)

(F + 40) * 5/9 - 40 = C

(C + 40) * 9/5 - 40 = F

Takes advantage of the fact that -40 F and -40 C are the same.

Remember it as (move origin -- convert -- move origin back).

I find it easier as a I don't have to remember precedence rules and the multiplication is obvious.

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