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Comment by dlcarrier | Hacker News Reader

Someone's actually using nF, instead of 0.022uF or 22,000pF.

I never understood why nF is less frequently used than uF or pF. To me, the logical thing is to express the capacity in units that are between 1 and 1000, like it's done with resistors and inductors.

I, too, had wondered; this seems particularly common in the US. My best theory is that in rough handwriting, n and u/μ can look pretty similar, and everyone sort of agreed to only use u to reduce ambiguity.

Another way to reduce ambiguity is to omit the suffix (unless it's p for picofarads) and let it be deduced from the numbers and symbols.

  47    ->  47nF (if next to plain double bar symbol with no polarity markings)
  47    ->  47μF (if next to polarized cap symbol)
  0.47  ->  0.47μF
  47p   ->  47pF
  10np  ->  np -> nonpolar -> electrolytic -> 10μF

I heard this, and have witnessed it. One of my first jobs included board rework. There were several times when I had to go ask if they wrote a uF or an nF. The response was always "obviously a uF", because nobody uses nF!

Nobody uses nF in, say, digital, where the only caps are power supply filter caps and bypass/decoupling caps next to ICs.

It's not customary to call a 0.1μF bypass cap 100 nF.

If someone wrote in 100 nF for a decoupling cap, I would tend suspect they are trying to discourage a substitution like 82nF or 150nF and want to find out why.

Whereas if it is 0.1μF, I know we can stick in a 0.2μF, if that's what we have on hand, and everything will be fine.

There's also a very old convention of using no units at all and assuming that "1" means uF.

nF is used a fair bit in audio circuits. Nanofarad-range capacitors, typically from 1 to 100 nF, often appear in filtering circuits. From time to time, values down to around 300 to 500 pF may appear. Anything around 100 pF or less is typically for dealing with unwanted high frequencies in some way. On the other end of the scale, microfarad-range caps in the signal path appear as couplers into low-ish impedances, where good bass response is required. For doing things" with the signal, other than passing it through with bass intact and filtering out RF, the workhorses tend to be caps that are nicely measured in nanos.

In terms of brevity on a schematic, it about breaks even for two-digit nF values and wider as in 22n vs .022, and only when we have a blanket rule that when units multipliers are omitted from capacitors, they are understood to be μ. 2n is shorter than .002; and harder .0047 is easier to misread as .047 compared to 4.7n.

NOTE: You will may across audio schematics which do not write the "n"! It is assumed that any capacitance which is an integer like 33, or a real number >= 1 like 2.7, is nanofarads! Together with the convention that fractional capacitances are implicitly microfarads, so that 0.47 is microfarads.

In my local brick-and-mortar electronics shop, packages of capacitors 1nF and higher tend to be labeled in nF if they are film caps or ceramics. Electrolytics use μF; a bag of 0.1μF electrolytics would be labelled that, and not 100 nF, but the same-valued film cap will probably be 100 nF.

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