Femto-

Femto (symbol f) is a unit prefix in the metric system denoting a factor of 10⁻¹⁵. It was added to the International System of Units (SI) in 1964.^[1] It is derived from the Danish word femten, meaning "fifteen".

Examples of use:

The HIV-1 virus has the mass of about 1 x 10⁻¹⁵ g or 1 fg.
A proton has a diameter of about 1.66 femtometres.
More examples are available here.

The femtometre shares the unit symbol (fm) with the older non-SI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.

SI prefixes
v
t
e
Prefix		Base 10	Decimal	English word		Adoption^{[nb 1]}	Etymology
Name	Symbol	Base 10	Decimal	Short scale	Long scale	Adoption^{[nb 1]}	Language	Derived word
yotta	Y	10²⁴	1000000000000000000000000	septillion	quadrillion	1991	Greek	eight^{[nb 2]}
zetta	Z	10²¹	1000000000000000000000	sextillion	trilliard	1991	Latin	seven^{[nb 2]}
exa	E	10¹⁸	1000000000000000000	quintillion	trillion	1975	Greek	six
peta	P^{[nb 3]}	10¹⁵	1000000000000000	quadrillion	billiard	1975	Greek	five^{[nb 2]}
tera	T	10¹²	1000000000000	trillion	billion	1960	Greek	four^{[nb 2]}, monster
giga	G	10⁹	1000000000	billion	milliard	1960	Greek	giant
mega	M	106	1000000	million		1873	Greek	great
kilo	k	103	1000	thousand		1795	Greek	thousand
hecto	h	102	100	hundred		1795	Greek	hundred
deca	da	101	10	ten		1795	Greek	ten
		100	1	one		–
deci	d	10−1	0.1	tenth		1795	Latin	ten
centi	c	10−2	0.01	hundredth		1795	Latin	hundred
milli	m	10−3	0.001	thousandth		1795	Latin	thousand
micro	μ	10−6	0.000001	millionth		1873	Greek	small
nano	n	10−9	0.000000001	billionth	milliardth	1960	Greek	dwarf
pico	p^{[nb 3]}	10−12	0.000000000001	trillionth	billionth	1960	Spanish	small
femto	f	10−15	0.000000000000001	quadrillionth	billiardth	1964	Danish	fifteen
atto	a	10−18	0.000000000000000001	quintillionth	trillionth	1964	Danish	eighteen
zepto	z	10−21	0.000000000000000000001	sextillionth	trilliardth	1991	Latin	seven^{[nb 2]}
yocto	y	10−24	0.000000000000000000000001	septillionth	quadrillionth	1991	Greek	eight^{[nb 2]}
^ Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873. ^ a b c d e f Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (derived word). ^ a b Uppercase P and lowercase p do not indicate the same power of ten for their respective multiples, rather powers of 15 and -12 respectively.

References[edit]

Look up femto- in Wiktionary, the free dictionary.

^ "Resolution 8 of the 12th CGPM". International Bureau of Weights and Measures. 1964. Retrieved 2020-05-14.

[2] Prefixes adopted before 1960 already existed before SI. The introduction of the CGS system was in 1873.

[similar-3] Part of the beginning of the prefix was modified from the word it was derived from, ex: "peta" (prefix) vs "penta" (derived word).

[bigpsmallp-4] Uppercase P and lowercase p do not indicate the same power of ten for their respective multiples, rather powers of 15 and -12 respectively.

[1] "Resolution 8 of the 12th CGPM". International Bureau of Weights and Measures. 1964. Retrieved 2020-05-14.

[1]