units

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 runnning 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 ×10m2 etc.

...except when units are used as an adjective. They then take a hypen:

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.

Standard prefixes for the SI units of measure
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.

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