Metric prefix

The SI prefixes for SI units are:

(Sub)multiple	Prefix	Symbol	Name (US and Canada)	Name (European)†
1024	yotta	Y	Septillion	Quadrillion
1021	zetta	Z	Sextillion	Thousand trillion (Trilliard)
1018	exa	E	Quintillion	Trillion
1015	peta	P	Quadrillion	Thousand billion (Billiard)
1012	tera	T	Trillion	Billion
109	giga	G	Billion	Thousand million (Milliard)
106	mega	M	Million	Million
103	kilo	k	Thousand	Thousand
102	hecto	h	Hundred	Hundred
101	deca	da	Ten	Ten
100	iso	i	Unit	Unit
10-1	deci	d	Tenth	Tenth
10-2	centi	c	Hundredth	Hundredth
10-3	milli	m	Thousandth	Thousandth
10-6	micro	μ	Millionth	Millionth
10-9	nano	n	Billionth	Milliardth
10-12	pico	p	Trillionth	Billionth
10-15	femto	f	Quadrillionth	Billiardth
10-18	atto	a	Quintillionth	Trillionth
10-21	zepto	z	Sextillionth	Trilliardth
10-24	yocto	y	Septillionth	Quadrillionth

Examples:

• 5 dm = 5 × 10^-2m = 5 × 0.01 m = 0.05 m
• 3 MW = 3 × 10⁶W = 3 × 1 000 000 W = 3 000 000 W

The prefix always takes precedence over any exponentiation. This means that 3 km² is equal to 3,000,000 m² and not to 3,000 m² (nor to 9,000,000 m²).

Prefixes where the exponent is divisible by three are recommended. Hence '100 metres' rather than 'one hectometre'. Notable exceptions include centimetre, hectare (hecto-are), centilitre, and 1 dm³ (equivalent to one litre).

Note that when used with non-SI units which typically come in powers of two, such as information and storage units like the bit and the byte, the prefixes traditionally had other values: K = 2¹⁰ = 1,024, M = 2²⁰ = 1,048,576, G = 2³⁰ = 1,073,741,824, T = 2⁴⁰ = 1,099,511,627,776 and P = 2⁵⁰ = 1,125,899,906,842,624. However, these prefixes retain their powers-of-1000 meanings when used to describe rates of data communication: 10-Mbps Ethernet runs at 10,000,000 bits per second, not 10,485,760 bits per second.

New binary prefixes have recently been adopted by the International Electrotechnical Commission, formed from the first syllable of the decimal prefix plus 'bi'. The symbol is the decimal symbol plus 'i'. So now, one kilobyte (1 kB) is 1000 bytes, whereas one kibibyte (1 KiB) is 1024 bytes. Likewise mebi, gibi, tebi, pebi, and exbi. For example, at 1 MB/s = 10⁶ bytes per second, it would take slightly longer than one second to transfer an object 1 MiB = 2²⁰ bytes in size.

See also Orders of magnitude.

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^† Britain, Ireland and Australia previously used the European number name conventions, but have now largely switched to US usage. See number names for the details.