SI Prefixes Explained, From Quecto to Quetta

An SI prefix is a word or symbol placed in front of a unit to multiply it by a power of ten. Kilo means one thousand, so a kilometer is 1000 meters and a kilowatt is 1000 watts. Milli means one thousandth, so a milligram is 0.001 gram.

The same prefixes work with every SI unit, which keeps very large and very small values short and readable. A wavelength of 0.000000550 m is easier to read as 550 nm.

The prefix deca is spelled deka in US NIST publications. Hecto, deca, deci and centi are used mostly with a few units, such as the hectare, the deciliter and the centimeter.

Prefix symbols are case-sensitive. All prefixes from mega upward use uppercase symbols (M, G, T, P, E, Z, Y, R, Q). Kilo and everything below use lowercase symbols (k, h, da, d, c, m, µ, n, p, f, a, z, y, r, q).

• m is milli and M is mega. A megawatt (MW) is 10⁹, one billion, times a milliwatt (mW).
• k is kilo, while uppercase K is the kelvin. Write km and kg, not Km or Kg.
• The symbol for micro is the Greek letter µ. Where it cannot be typed, "u" is sometimes used informally, but µ is the correct symbol.

• Use only one prefix at a time. Write nanometer (nm), not millimicrometer.
• A prefix and its unit form a single symbol, and an exponent applies to the whole symbol: cm² means (0.01 m)², which is 0.0001 m², not 0.01 m².
• Write the prefix symbol directly in front of the unit symbol with no space: mm, kPa, GHz.
• A prefix is never used on its own. Write 5 kg, not 5 k.

Converting between prefixes is a matter of subtracting exponents. Kilo to milli is 10³ ÷ 10⁻³ = 10⁶, so 1 kilo-unit is one million milli-units. Giga to mega is a factor of 1000, and micro to nano is a factor of 1000.

The kilogram is the only SI base unit whose name already contains a prefix. For historical reasons, prefixes for mass are attached to the gram, not to the kilogram.

So one millionth of a kilogram is a milligram (mg), not a microkilogram. One thousand kilograms is a megagram (Mg), more commonly called the tonne (t).

In 2022, the 27th General Conference on Weights and Measures (CGPM) added four prefixes: ronna (R, 10²⁷), quetta (Q, 10³⁰), ronto (r, 10⁻²⁷) and quecto (q, 10⁻³⁰). They were the first new SI prefixes since zetta, zepto, yotta and yocto were adopted in 1991.

These prefixes make extreme values easier to write. The mass of the Earth, about 6 × 10²⁴ kg, is about 6 ronnagrams (Rg). In everyday work, prefixes from pico to tera cover almost everything.

In computing, memory sizes come in powers of two. The International Electrotechnical Commission (IEC) defines binary prefixes for these: kibi (Ki) = 2¹⁰ = 1024, mebi (Mi) = 2²⁰ = 1,048,576, gibi (Gi) = 2³⁰ and tebi (Ti) = 2⁴⁰. They are not SI prefixes.

• A kilobyte (kB) is 1000 bytes; a kibibyte (KiB) is 1024 bytes.
• A gibibyte (GiB) is 1.073741824 gigabytes (GB).
• A drive sold as 1 TB holds 0.9095 TiB. That is why an operating system that counts in binary units shows less space than the label.

Some software still writes KB, MB and GB when it means the binary values. When the difference matters, check which one is meant.