The femtometre (American spelling femtometer, symbol fm derived from the Danish and Norwegian word femten, "fifteen"+Ancient Greek: μέτρον, metrοn, "unit of measurement") is an SI unit of length equal to 10−15 metres, which means a quadrillionth of one. This distance can also be called a fermi and was so named in honour of physicist Enrico Fermi, as it is a typical length-scale of nuclear physics.
| Definition | Relation to SI units |
|---|---|
≡ 1×10−15 m |
| pace | gigameter | league (land) | zettameter | rod | finger | spat | hand |
|---|---|---|---|---|---|---|---|
| 1 | 7.62 × 10-10 | 0.0001578 | 7.62 × 10-22 | 0.1515152 | 34.2857143 | 7.62 × 10-13 | 7.5 |
| 1,312,335,958.0052 | 1 | 207,125.1035626 | 1.0 × 10-12 | 198,838,781.51595 | 44,994,375,703.037 | 0.001 | 9,842,519,685.0394 |
| 6,335.9580052 | 4.828 × 10-6 | 1 | 4.828 × 10-18 | 959.9936372 | 217,232.8458943 | 4.828 × 10-9 | 47,519.6850394 |
| 1.3123359580052 × 1021 | 1 000 000 000 000 | 2.0712510356255 × 1017 | 1 | 1.9883878151595 × 1020 | 4.4994375703037 × 1022 | 1 000 000 000 | 9.8425196850394 × 1021 |
| 6.6 | 5.0292 × 10-9 | 0.0010417 | 5.0292 × 10-21 | 1 | 226.2857143 | 5.0292 × 10-12 | 49.5 |
| 0.0291667 | 2.2225 × 10-11 | 4.6033554266777 × 10-6 | 2.2225 × 10-23 | 0.0044192 | 1 | 2.2225 × 10-14 | 0.21875 |
| 1,312,335,958,005.2 | 1 000 | 207,125,103.56255 | 1.0 × 10-9 | 198,838,781,515.95 | 44 994 375 703 037 | 1 | 9,842,519,685,039.4 |
| 0.1333333 | 1.016 × 10-10 | 2.1043910521955 × 10-5 | 1.016 × 10-22 | 0.0202020 | 4.5714286 | 1.016 × 10-13 | 1 |
| Symbol | Definition |
|---|---|
| ≡ | exactly equal |
| ≈ | approximately equal to |
| = | equal to |
| digits | indicates that digits repeat infinitely (e.g. 8.294 369 corresponds to 8.294 369 369 369 369 …) |
| 1 fermi | = 1.0E-15 meters |
| 1.5 fermi | = 1.5E-15 meters |
| 2 fermi | = 2.0E-15 meters |
| 2.5 fermi | = 2.5E-15 meters |
| 3 fermi | = 3.0E-15 meters |