The metric system and unit prefixes (milli, kilo, mega…)
Metric system prefixes, from nano to giga, and how to quickly convert between them.
Common prefixes
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| giga | G | ×10⁹ | GB (gigabyte) |
| mega | M | ×10⁶ | MHz (megahertz) |
| kilo | k | ×10³ | km (kilometer) |
| (base unit) | — | ×10⁰ | m, g, s, L |
| centi | c | ×10⁻² | cm (centimeter) |
| milli | m | ×10⁻³ | ms (millisecond) |
| micro | µ | ×10⁻⁶ | µm (micrometer) |
| nano | n | ×10⁻⁹ | ns (nanosecond) |
Converting between prefixes
Each step to the right in the table divides by 1000 (or 10, 100 depending on the prefix); each step to the left multiplies by the same amount.
1 km = 1,000 m 1 m = 100 cm = 1,000 mm 1 g = 1,000 mg 1 GHz = 1,000 MHz = 1,000,000 kHz
It's more reliable to just memorize the table above, from largest to smallest, than to rely on a risky mnemonic trick.
A computing quirk
In computing, "kilo/mega/giga" sometimes mean powers of 1024 (2¹⁰) instead of 1000, especially for memory (RAM). Official binary prefixes exist to remove the ambiguity (kibi Ki, mebi Mi, gibi Gi), but remain rarely used in everyday language.
| Unit | Decimal (SI) value | Binary value (memory usage) |
|---|---|---|
| 1 kB / 1 KiB | 1,000 bytes | 1,024 bytes |
| 1 MB / 1 MiB | 1,000,000 bytes | 1,048,576 bytes |
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