SI Unit Prefixes | Metric prefixes

Twenty prefixes are used by the SI sytem of measurement (the metric system) to form decimal multiples and submultiples of the base and derived SI units.
See our page on SI Units for more information on the base and derived units.
Many of these prefixes are in every day use in engineering reports, control system displays etc. e.g. Megawatt uses the SI prefix mega, and kilometer uses the SI prefix kilo.
Care has to be taken when using engineering calculations as they often require the base or derived unit to be used, e.g. instead of 3 Megawatts formulae often require 3,000,000 watts to be used.

What are the prefixes for SI units?

A complete list of the 20 SI prefixes, and the symbol used to denote each prefix is shown below:

SI Prefixes
Factor	Name	Symbol
10²⁴	yotta	Y
10²¹	zetta	Z
10¹⁸	exa	E
10¹⁵	peta	P
10¹²	tera	T
10⁹	giga	G
10⁶	mega	M
10³	kilo	k
10²	hecto	h
10¹	deka	da
10^-1	deci	d
10^-2	centi	c
10^-3	milli	m
10^-6	micro	µ
10^-9	nano	n
10^-12	pico	p
10^-15	femto	f
10^-18	atto	a
10^-21	zepto	z
10^-24	yocto	y

Points to Note When Using a Prefix to an SI Unit

- The kilogram is the only SI unit with a prefix as part of its name and symbol i.e. kg. Under the SI convention multiple prefixes may not be used, therefore when using kilogram the prefix names above are used with the unit name "gram" and the prefix symbols are used with the unit symbol "g."
- Apart from the kilogram, any SI prefix may be used with any SI unit, including the degree Celsius and its symbol °C. Note however, it is unusual to use prefixes with Celsius.
- The SI prefixes represent powers of 10, therefore their use for binary multiples e.g. one kilobit, or 1 kbit, which is actually 1024 bits is not strictly correct. However, the International Electrotechnical Commission (IEC) has adopted this system of prefixing for use in information technology.

Return to Technical Toolbox index