SI prefixes explained

How prefixes work

A prefix scales the unit that follows it. Kilo means 1,000, so one kilometre is 1,000 metres. Milli means one thousandth, so one millimetre is 0.001 metre. The unit retains the same physical meaning; only its scale changes.

Prefix symbols are case-sensitive. M means mega, or one million, while m means milli, or one thousandth. The symbol for kilo is lowercase k. Writing the correct case prevents enormous errors.

Everyday SI prefixes

These prefixes cover most daily, scientific, and technical measurements.

• giga (G) = 10⁹ = 1,000,000,000
• mega (M) = 10⁶ = 1,000,000
• kilo (k) = 10³ = 1,000
• centi (c) = 10⁻² = 0.01
• milli (m) = 10⁻³ = 0.001
• micro (µ) = 10⁻⁶ = 0.000001
• nano (n) = 10⁻⁹ = 0.000000001

Converting between prefixed units

Compare the powers of ten and multiply by their difference. A centimetre represents 10⁻² metre and a millimetre represents 10⁻³ metre. The difference is one power of ten, so one centimetre equals 10 millimetres.

For squared and cubed measurements, the prefix is also squared or cubed. Because one metre equals 100 centimetres, one square metre equals 100², or 10,000 square centimetres—not 100 square centimetres.

New prefixes for extreme values

The SI added ronna and quetta for 10²⁷ and 10³⁰, plus ronto and quecto for 10⁻²⁷ and 10⁻³⁰. Most people will rarely need them, but they provide consistent names for quantities at the edges of modern science and data measurement.