SI prefixes and scientific notation explained
Read all 24 SI prefixes, distinguish uppercase and lowercase symbols, and convert decimal powers without confusing a measurement with its unit.
Updated · How we checkA prefix multiplies a unit by a defined power of ten. Case matters: m means milli (10⁻³), while M means mega (10⁶).
A reliable conversion method
First write each unit as a multiple of a shared reference unit. Multiply the input by its unit factor and divide by the target factor. For a difference of six powers of ten, the numerical value changes by a factor of one million.
The smallest and largest prefixes
The SI prefix list extends from quecto (q), meaning 10⁻³⁰, to quetta (Q), meaning 10³⁰. The 2022 additions were ronto, ronna, quecto and quetta. These prefixes describe scales; they do not establish measurement accuracy.
Mass and compound units
Mass prefixes attach to gram, not kilogram: a milligram is 10⁻³ grams and 10⁻⁶ kilograms. In compound units, identify the exact component carrying the prefix. A square millimeter squares both the millimeter unit and its factor.
Put the formula into practice
Express 2.5 milliamperes in microamperes.
2.5 mA = 2,500 µA. The current has not changed; only the reporting unit has.
References
Definitions and method references. Example rates and prices on this page are editable assumptions.