SI prefix

SI prefixes for SI units are:

(Sub)multiple	Prefix	Symbol	Name (US and Canada)	Name (Europe)†
1024	yotta	Y	Septillion	Quadrillion
1021	zetta	Z	Sextillion	Trilliard
1018	exa	E	Quintillion	Trillion
1015	peta	P	Quadrillion	Billiard
1012	tera	T	Trillion	Billion
109	giga	G	Billion	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 x 10^-2m = 5 x 0.01 m = 0.05 m
• 3 MW = 3 x 10⁶W = 3 x 1 000 000 W = 3 000 000 W

Note that when used with non-SI units, prefices occasionally have other values. For instance, when referring to units of storage like bytes, kilo means 2¹⁰=1024 rather than 1000.

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? (being equivalent to 1m³), and 1 dm³ (equivalent to one litre, sometimes referred to as a 'dumphrey').

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^† In Britain and Ireland the names that end in "-iard" and "-iardth" are not used. Instead for example 10⁹ is referred to as a "thousand million", though the U.S. naming is also sometimes used.