Definition · Plain-language
Metric prefixes
Metric prefixes are standard multipliers — such as kilo-, centi-, milli- and mega- — that attach to a unit to scale it up or down by a power of ten.
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Multipliers that attach to any unit
A metric prefix is a word-part placed in front of a unit to multiply or divide it by a power of ten. Kilo- means a thousand, so a kilometre is a thousand metres and a kilogram a thousand grams; milli- means a thousandth, so a millimetre is a thousandth of a metre. Crucially, the prefix carries the same meaning whatever unit it joins — kilo- is always a thousand, whether of metres, grams, watts or bytes. This universality is the heart of the system’s economy: learn the prefixes once and they apply everywhere.
The common prefixes, large and small
For everyday and scientific use the workhorse prefixes climb and descend in steps of a thousand. Going up: kilo- (10³), mega- (10⁶), giga- (10⁹) and tera- (10¹²). Going down: milli- (10⁻³), micro- (10⁻⁶), nano- (10⁻⁹) and pico- (10⁻¹²). Two smaller steps, centi- (a hundredth) and deci- (a tenth), survive mainly in the centimetre and in litres. The prefix symbols matter too: a capital M is mega while a lowercase m is milli, so case is not optional — Mg and mg differ by a factor of a billion.
Using prefixes in conversions
Because each prefix is a clean power of ten, converting between prefixed units is a matter of counting decimal places. To turn 2.5 kilometres into metres you multiply by 1,000, giving 2,500 metres; to turn 4,000 milligrams into grams you divide by 1,000, giving 4 grams. The safest habit is to write the prefix as its power of ten and let the exponents do the work, which avoids the slips that come from juggling zeros. Scientific notation pairs naturally with prefixes for exactly this reason.
Key facts
At a glance
- Definition: multipliers that scale a unit by a power of ten
- Kilo-: ×1,000 (10³) — kilometre, kilogram
- Centi-/milli-: ÷100 and ÷1,000 — centimetre, millilitre
- Mega-/giga-: ×10⁶ and ×10⁹ — megawatt, gigabyte
- Micro-/nano-: ÷10⁶ and ÷10⁹ — microsecond, nanometre
- Case matters: M (mega) and m (milli) differ by a factor of a billion
Common misconceptions
What people often get wrong
Often heard: Metric prefixes change meaning depending on the unit they join.
Actually: They do not. Kilo- always means a thousand, whether it is a kilometre, kilogram or kilojoule. That consistency is the whole point of the prefix system.
Often heard: The capital and lowercase versions of a prefix symbol are interchangeable.
Actually: Case is significant. A capital M means mega (a million) and a lowercase m means milli (a thousandth), so Mg and mg differ by a factor of a billion.
Often heard: Centi- is the standard prefix for everything smaller than a unit.
Actually: Scientific prefixes generally step in thousands — milli-, micro-, nano-. Centi- (a hundredth) survives mainly in the centimetre; most small measurements use milli- or smaller.
