Femtometer (fm)

One quadrillionth of a meter, also called a fermi. The scale of atomic nuclei; a proton is roughly 0.84 fm in radius.

1 femtometer = 1×10⁻¹⁵ meters (exactly).

ValueFromTo

1 femtometer = 1×10⁻¹⁵ meters

About the femtometer

Robert Hofstadter coined the nickname fermi for this length in a 1956 paper in Reviews of Modern Physics, honoring Enrico Fermi. The formal name comes from femto, which Wikipedia traces to the Danish and Norwegian word femten, meaning fifteen, a nod to the factor of 10 to the minus 15. The prefix entered the SI in 1964, and Hofstadter also used the term in his 1961 Nobel Lecture.

One femtometer is a quadrillionth of a meter, or 1,000 picometers. The unit suits atomic nuclei: Wikipedia gives a proton charge radius of about 0.841 fm and a gold nucleus radius of about 8.45 fm, so a heavy nucleus is roughly ten times wider than a proton. The 0.841 fm figure from that article is the one used here. Cross-sections in the same field are measured in barns, and one barn equals 100 square femtometers.

Sources: Wikipedia: Femtometre; Wikipedia: Metric prefix; NIST: metric (SI) prefixes

Femtometer conversion chart

1 femtometer expressed in every other length unit:

Unit	1 femtometer
Meters	1×10⁻¹⁵
Kilometers	1×10⁻¹⁸
Centimeters	1×10⁻¹³
Millimeters	1×10⁻¹²
Inches	3.937007874×10⁻¹⁴
Feet	3.280839895×10⁻¹⁵
Yards	1.093613298×10⁻¹⁵
Miles	6.213711922×10⁻¹⁹
Micrometers	1×10⁻⁹
Nanometers	0.000001
Nautical Miles	5.399568035×10⁻¹⁹
Pixels (CSS)	3.779527559×10⁻¹²
Light-years	1.057000834×10⁻³¹
Picometers	0.001
Decimeters	1×10⁻¹⁴
Hectometers	1×10⁻¹⁷
Decameters	1×10⁻¹⁶
Thou	3.937007874×10⁻¹¹
Points	2.834645669×10⁻¹²
Picas	2.362204724×10⁻¹³
Twips	5.669291339×10⁻¹¹
Astronomical Units	6.684587122×10⁻²⁷
Light-seconds	3.335640952×10⁻²⁴
Light-minutes	5.559401587×10⁻²⁶
Light-hours	9.265669311×10⁻²⁸
Parsecs	3.240779289×10⁻³²
Megaparsecs	3.240779289×10⁻³⁸
Angstroms	0.00001
Fathoms	5.468066492×10⁻¹⁶
Cable Lengths	5.399568035×10⁻¹⁸
Leagues	2.071237307×10⁻¹⁹
Nautical Leagues	1.799856012×10⁻¹⁹
Furlongs	4.970969538×10⁻¹⁸
Chains	4.970969538×10⁻¹⁷
Rods	1.988387815×10⁻¹⁶
Links	4.970969538×10⁻¹⁵
Hands	9.842519685×10⁻¹⁵
Barleycorns	1.181102362×10⁻¹³
Palms	1.312335958×10⁻¹⁴
Spans	4.374453193×10⁻¹⁵
Cubits	2.187226597×10⁻¹⁵
Ells	8.748906387×10⁻¹⁶
US Survey Feet	3.280833333×10⁻¹⁵
US Survey Miles	6.213699495×10⁻¹⁹
Stadia	5.208333333×10⁻¹⁸
Roman Inches	4.06504065×10⁻¹⁴
Roman Feet	3.378378378×10⁻¹⁵
Roman Cubits	2.252252252×10⁻¹⁵
Roman Paces	6.756756757×10⁻¹⁶
Roman Stadia	5.405405405×10⁻¹⁸
Roman Miles	6.756756757×10⁻¹⁹
Roman Leagues	4.504504505×10⁻¹⁹
Greek Feet	3.125×10⁻¹⁵
Royal Cubits	1.904761905×10⁻¹⁵
Egyptian Palms	1.333333333×10⁻¹⁴
Egyptian Fingers	5.333333333×10⁻¹⁴
Khet	1.904761905×10⁻¹⁷
Biblical Cubits	1.851851852×10⁻¹⁵
Sabbath Days' Journeys	9.259259259×10⁻¹⁹
Sun	3.3×10⁻¹⁴
Shaku	3.3×10⁻¹⁵
Ken	5.5×10⁻¹⁶
Jō	3.3×10⁻¹⁶
Chō	9.166666667×10⁻¹⁸
Ri (Japanese)	2.546296296×10⁻¹⁹
Fen (Chinese)	3×10⁻¹³
Cun (Chinese)	3×10⁻¹⁴
Chi (Chinese)	3×10⁻¹⁵
Zhang (Chinese)	3×10⁻¹⁶
Li (Chinese)	2×10⁻¹⁸
Vershoks	2.249718785×10⁻¹⁴
Arshins	1.406074241×10⁻¹⁵
Sazhens	4.686914136×10⁻¹⁶
Versts	9.373828271×10⁻¹⁹
Lignes	4.43296×10⁻¹³
Pouces	3.694133333×10⁻¹⁴
Pieds Du Roi	3.078444444×10⁻¹⁵
Toises	5.130740741×10⁻¹⁶
Lieues	2.56537037×10⁻¹⁹
Varas	1.196315349×10⁻¹⁵
Leguas	1.794473023×10⁻¹⁹
Wa (Thai)	5×10⁻¹⁶
Sen (Thai)	2.5×10⁻¹⁷
Scandinavian Mil	1×10⁻¹⁹

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