Femto-

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This article describes the SI prefix. For the manga character, see Berserk (manga).

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Femto- is an SI prefix (symbol f) in the SI system of units denoting a factor of 10⁻¹⁵ or 0.000 000 000 000 001. Adopted in 1964 by the 12th Conférence Générale des Poids et Mesures ^[1], it comes from the Danish or Norwegian femten, Old Norse fimmtān, meaning fifteen.

Examples of units:

One femtosecond = 10⁻¹⁵ s
One femtojoule = 10⁻¹⁵ J

Example:

a proton has a diameter of about 1.6 to 1.7 femptometre

Note that the femtometre (10⁻¹⁵ of a metre) shares the same unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, in honour of Enrico Fermi is often encountered in nuclear physics that is characteristic of this scale.

SI prefixes
1000ⁿ	10ⁿ	Prefix	Symbol	Since^[1]	Short scale	Long scale	Decimal equivalent in SI writing style
1000⁸	10²⁴	yotta-	Y	1991	Septillion	Quadrillion	1 000 000 000 000 000 000 000 000
1000⁷	10²¹	zetta-	Z	1991	Sextillion	Trilliard (thousand trillion)	1 000 000 000 000 000 000 000
1000⁶	10¹⁸	exa-	E	1975	Quintillion	Trillion	1 000 000 000 000 000 000
1000⁵	10¹⁵	peta-	P	1975	Quadrillion	Billiard (thousand billion)	1 000 000 000 000 000
1000⁴	10¹²	tera-	T	1960	Trillion	Billion	1 000 000 000 000
1000³	10⁹	giga-	G	1960	Billion	Milliard (thousand million)	1 000 000 000
1000²	10⁶	mega-	M	1960	Million		1 000 000
1000¹	10³	kilo-	k	1795	Thousand		1 000
1000^2/3	10²	hecto-	h	1795	Hundred		100
1000^1/3	10¹	deca-	da	1795	Ten		10
1000⁰	10⁰	(none)	(none)	NA	One		1
1000^−1/3	10⁻¹	deci-	d	1795	Tenth		0.1
1000^−2/3	10⁻²	centi-	c	1795	Hundredth		0.01
1000⁻¹	10⁻³	milli-	m	1795	Thousandth		0.001
1000⁻²	10⁻⁶	micro-	µ	1960^[2]	Millionth		0.000 001
1000⁻³	10⁻⁹	nano-	n	1960	Billionth	Milliardth	0.000 000 001
1000⁻⁴	10⁻¹²	pico-	p	1960	Trillionth	Billionth	0.000 000 000 001
1000⁻⁵	10⁻¹⁵	femto-	f	1964	Quadrillionth	Billiardth	0.000 000 000 000 001
1000⁻⁶	10⁻¹⁸	atto-	a	1964	Quintillionth	Trillionth	0.000 000 000 000 000 001
1000⁻⁷	10⁻²¹	zepto-	z	1991	Sextillionth	Trilliardth	0.000 000 000 000 000 000 001
1000⁻⁸	10⁻²⁴	yocto-	y	1991	Septillionth	Quadrillionth	0.000 000 000 000 000 000 000 001
Notes: 1. The 1795 dates identify prefixes in use since the metric system was introduced. The other dates are not necessarily dates of first use, but rather the date of recognition by a resolution of the CGPM, which first met in 1889. 2. The micron was earlier recognized by the CGPM in 1948; that decision was abrogated in 1967-68.