International System (SI) prefixes

What it's for

SI prefixes multiply or divide units by powers of ten: kilo = thousand, mega = one million, milli = one thousandth. Useful in physics, computing and everyday life.

Example

1 km = 1000 m, 1 MB ≈ one million bytes, 1 ms = one thousandth of a second.

The prefixes in order, and the ones people forget

Going up: kilo (thousand), mega (million), giga (billion), tera, then peta, exa, zetta, yotta and the two added in 2022, ronna and quetta. Going down: milli (a thousandth), micro (a millionth), nano, pico, then femto, atto, zepto, yocto and the new ronto and quecto. From kilo upwards and from milli downwards you move in steps of a thousand: they're the powers of ten that are multiples of three, and that makes the arithmetic easy.

In between there are four that get little use and are often needed: hecto (hundred), deca (ten), deci (a tenth) and centi (a hundredth). They're the only ones that don't follow the thousand step, which is why the hectogram, the decilitre and the centimetre feel out of system without being so.

Capital letters aren't a cosmetic detail

M is mega, a million; m is milli, a thousandth: between the two there are nine orders of magnitude, and they're the upper and lower case of the same letter. The rule is sharp: from kilo upwards the symbols are capitals (M, G, T), from kilo downwards lower case (m, µ, n, p), and kilo itself is the historical exception, with a lower-case k. Writing «Mb» instead of «mb» in a technical report is a mistake that changes the meaning of the sentence.

The computing case

Computer memory is built in powers of two, not of ten, which is why computing's «kilo» meant 1024 rather than 1000 for decades. From there came the confusion that still lasts: disks are sold counting by a thousand, Windows counts by 1024, and the official names for the second way (kibi, mebi, gibi) are used by almost nobody. The difference reaches 10% on terabytes: it's discussed in Byte converter.

Nearby tools

To actually convert between units there are Length converter, Weight converter and Energy converter. For powers of ten there is Power calculator and for the inverse operation Logarithm.