Zoë Blade's notebook

SI unit

An SI unit (short for Système International) is a very thoroughly standardised and understood means of measuring something.

For the purposes of talking about music and microcomputers (two of the very few things I know anything about), the two SI units to concern ourselves with most are the base unit of seconds, and the derived unit of Hertz, the latter literally being 1 ÷ seconds.

In the context of mechanical keyboards, you'll see centinewtons (hundredths of a newton) mentioned. Millimetres are quite ubiquitous throughout this site, such as when measuring phone jacks.

SI prefix

To reduce verbosity, SI units can be prefixed to multiply or divide them by multiples of a thousand. At the scales we're dealing with here, we only need to concern ourselves with a handful of SI prefixes:

Prefix Abbreviation Amount
nano n ×0.000000001
micro μ ×0.000001
milli m ×0.001
kilo k ×1,000
mega M ×1,000,000
giga G ×1,000,000,000

For example, if a periodic waveform repeats a thousand times every second, we can say it has a frequency of 1 kilohertz, or 1 kHz for short, and that each cycle lasts 1 millisecond, or 1 ms for short. If it repeats a million times every second, its frequency is 1 MHz, and each cycle lasts 1 μs.

Binary prefix

For computers in particular, their memory is almost always binary, so instead of measuring bits, nibbles, bytes, or words in multiples of 1,000 (×103), we measure them in multiples of 1,024 (×210).

Prefix Abbreviation Amount
kilo K ×1,024
mega M ×1,048,576
giga G ×1,073,741,824
tera T ×1,099,511,627,776

This is a pretty substantial abuse of SI prefixes, from the uppercase K (which is, admittedly, neatly consistent) to the rounding off in binary.

In order to disambiguate binary prefixes from their decimal counterparts, it's now recommended to replace their last syllable with bi for binary, and add a lowercase letter i between the prefix abbreviation and unit of measurement, as in KiB for kibibyte rather than KB for kilobyte.

Prefix Abbreviation Amount
kibi Ki ×1,024
mebi Mi ×1,048,576
gibi Gi ×1,073,741,824
tebi Ti ×1,099,511,627,776

To my knowledge, this hasn't yet gained much traction, as historically people in the computer industry have been long accustomed to appropriating the decimal prefixes as they are.

At any rate, SI prefixes, along with SI units themselves, are wonderfully convenient.

Standards: ISBN | SI unit | Useful Unicode characters

SI units: Bit | Hertz