All SI units are lower case when spelled out: metre (m), kilogram (kg), second (s), ampere (A), kelvin (K), candela (cd), mole (mol).
Note: The tesla is the SI-derived unit of magnetic flux density. It has a lower-case "t" when spelled out, though its symbol is an upper-case "T".
Spell them out on first mention, then use the symbol.
I weigh 75 kilograms. Last month I gained 4 kg after I stopped running 5 kilometres every morning.
Always put a space between the digit and the unit when the unit is expressed as a symbol:
2 km, 16.87 kg, 1000 cd, 8.2 ×104 m2 etc.
...except when units are used as an adjective. They then take a hyphen:
Today I ran 6 kilometres. I hope to run a 10-km marathon next year.
For rates, use the form m/s2 not m-1s-1
The acceleration due to gravity on Earth is 9.81 m/s2.
See numbers and percentage
PREFIXES
Can be added to a unit to denote a multiple of that unit. When spelled out, prefixes do not take a hyphen:
We used 5 petavolts of electricity to get the accelerator working.
The prefixes are never combined: a millionth of a metre is a micrometre not a millimillimetre.
| Multiples | Name | deca- | hecto- | kilo- | mega- | giga- | tera- | peta- | exa- | zetta- | yotta- | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Symbol | da | h | k | M | G | T | P | E | Z | Y | ||
| Factor | 100 | 101 | 102 | 103 | 106 | 109 | 1012 | 1015 | 1018 | 1021 | 1024 | |
| Fractions | Name | deci- | centi- | milli- | micro- | nano- | pico- | femto- | atto- | zepto- | yocto- | |
| Symbol | d | c | m | μ | n | p | f | a | z | y | ||
| Factor | 100 | 10−1 | 10−2 | 10−3 | 10−6 | 10−9 | 10−12 | 10−15 | 10−18 | 10−21 | 10−24 | |
Common units with prefixes used at CERN include megaelectronvolt (MeV, 106 electronvolts), gigaelectronvolt (GeV, 109 electronvolts) and teraelectronvolt (TeV, 1012 electronvolts).
See numbers.