Metric Prefixes Explained: From Nano to Giga With Everyday Examples

Each prefix multiplies the unit it is attached to by a power of ten. Above the base unit the common steps go up by a factor of 1,000; below it the same pattern repeats with thousandths. The centi, deci, deca and hecto prefixes fill in the steps of ten near the base unit and are mostly used for length, volume and area.

Prefix symbols are case-sensitive, and getting them wrong changes the meaning enormously. “M” is mega (a million) while “m” is milli (a thousandth) — a factor of a billion apart. A power station producing 500 MW and a device drawing 500 mW are very different things.

The kilo symbol is a lowercase “k”, so kilometres are written km, not Km or KM. Units named after people take a capital letter in their symbol (W for watt, after James Watt; N for newton) but are written in lowercase when spelled out. The micro symbol is the Greek letter mu, µ; when that character is unavailable people sometimes substitute a “u”, as in “ug” for micrograms.

In almost every case the base unit has no prefix: the metre, the second, the ampere. Mass is the exception. The SI base unit of mass is the kilogram, which already contains a prefix. Prefixes are therefore applied to the gram instead — you write milligram, not microkilogram. This quirk dates from the 1790s, when the original “grave” was renamed and the kilogram, rather than the tiny gram, became the practical standard.

Computing introduced a confusion of its own. Computer memory naturally comes in powers of two, and 2¹⁰ = 1,024 is close to 1,000, so for decades “kilobyte” was used loosely to mean either 1,000 or 1,024 bytes. Drive manufacturers use the decimal SI meaning: one terabyte is exactly 1,000,000,000,000 bytes. Some operating systems, notably Windows, divide by powers of 1,024 instead and then label the result “GB”.

One trillion bytes divided by 1,024³ is about 931, so a brand-new 1 TB drive appears as roughly “931 GB”. Nothing is missing — the same quantity is just being reported in two different units. To remove the ambiguity, the International Electrotechnical Commission introduced binary prefixes in 1998: kibibyte (KiB, 1,024 bytes), mebibyte (MiB) and gibibyte (GiB). They are standard in technical documentation, though everyday marketing rarely uses them.

Science keeps needing bigger and smaller numbers. In 2022 the General Conference on Weights and Measures added four new prefixes, the first additions since 1991: ronna (10²⁷) and quetta (10³⁰) for the very large, and ronto (10⁻²⁷) and quecto (10⁻³⁰) for the very small. They were partly prompted by the growth of global data storage, which is heading toward the yottabyte range, and they let scientists express quantities like the mass of the Earth — about 6 ronnagrams — compactly.

Because every step is a power of ten, converting between prefixed units is a matter of moving the decimal point. From kilometres to metres, move it three places right (2.5 km = 2,500 m). From milligrams to grams, move it three places left (250 mg = 0.25 g). For units of area and volume the step is squared or cubed: one square metre is 10,000 square centimetres, and one cubic metre is 1,000 litres.