| span | 1 span | 2 span | 3 span | 4 span | 5 span | 6 span | 7 span | 8 span | 9 span | 10 span | 11 span | 12 span | 13 span | 14 span | 15 span | 16 span | 17 span | 18 span | 19 span | 20 span | 21 span | 22 span | 23 span | 24 span | 25 span | 26 span | 27 span | 28 span | 29 span | 30 span | 31 span | 32 span | 33 span | 34 span | 35 span | 36 span | 37 span | 38 span | 39 span | 40 span | 41 span | 42 span | 43 span | 44 span | 45 span | 46 span | 47 span | 48 span | 49 span | 50 span | 51 span | 52 span | 53 span | 54 span | 55 span | 56 span | 57 span | 58 span | 59 span | 60 span | 61 span | 62 span | 63 span | 64 span | 65 span | 66 span | 67 span | 68 span | 69 span | 70 span | 71 span | 72 span | 73 span | 74 span | 75 span | 76 span | 77 span | 78 span | 79 span | 80 span | 81 span | 82 span | 83 span | 84 span | 85 span | 86 span | 87 span | 88 span | 89 span | 90 span | 91 span | 92 span | 93 span | 94 span | 95 span | 96 span | 97 span | 98 span | 99 span | 100 span |
| fermi | 2.286 × 1014 fm | 4.572 × 1014 fm | 6.858 × 1014 fm | 9.144 × 1014 fm | 1.143 × 1015 fm | 1.3716 × 1015 fm | 1.6002 × 1015 fm | 1.8288 × 1015 fm | 2.0574 × 1015 fm | 2.286 × 1015 fm | 2.5146 × 1015 fm | 2.7432 × 1015 fm | 2.9718 × 1015 fm | 3.2004 × 1015 fm | 3.429 × 1015 fm | 3.6576 × 1015 fm | 3.8862 × 1015 fm | 4.1148 × 1015 fm | 4.3434 × 1015 fm | 4.572 × 1015 fm | 4.8006 × 1015 fm | 5.0292 × 1015 fm | 5.2578 × 1015 fm | 5.4864 × 1015 fm | 5.715 × 1015 fm | 5.9436 × 1015 fm | 6.1722 × 1015 fm | 6.4008 × 1015 fm | 6.6294 × 1015 fm | 6.858 × 1015 fm | 7.0866 × 1015 fm | 7.3152 × 1015 fm | 7.5438 × 1015 fm | 7.7724 × 1015 fm | 8.001 × 1015 fm | 8.2296 × 1015 fm | 8.4582 × 1015 fm | 8.6868 × 1015 fm | 8.9154 × 1015 fm | 9.144 × 1015 fm | 9.3726 × 1015 fm | 9.6012 × 1015 fm | 9.8298 × 1015 fm | 1.00584 × 1016 fm | 1.0287 × 1016 fm | 1.05156 × 1016 fm | 1.07442 × 1016 fm | 1.09728 × 1016 fm | 1.12014 × 1016 fm | 1.143 × 1016 fm | 1.16586 × 1016 fm | 1.18872 × 1016 fm | 1.21158 × 1016 fm | 1.23444 × 1016 fm | 1.2573 × 1016 fm | 1.28016 × 1016 fm | 1.30302 × 1016 fm | 1.32588 × 1016 fm | 1.34874 × 1016 fm | 1.3716 × 1016 fm | 1.39446 × 1016 fm | 1.41732 × 1016 fm | 1.44018 × 1016 fm | 1.46304 × 1016 fm | 1.4859 × 1016 fm | 1.50876 × 1016 fm | 1.53162 × 1016 fm | 1.55448 × 1016 fm | 1.57734 × 1016 fm | 1.6002 × 1016 fm | 1.62306 × 1016 fm | 1.64592 × 1016 fm | 1.66878 × 1016 fm | 1.69164 × 1016 fm | 1.7145 × 1016 fm | 1.73736 × 1016 fm | 1.76022 × 1016 fm | 1.78308 × 1016 fm | 1.80594 × 1016 fm | 1.8288 × 1016 fm | 1.85166 × 1016 fm | 1.87452 × 1016 fm | 1.89738 × 1016 fm | 1.92024 × 1016 fm | 1.9431 × 1016 fm | 1.96596 × 1016 fm | 1.98882 × 1016 fm | 2.01168 × 1016 fm | 2.03454 × 1016 fm | 2.0574 × 1016 fm | 2.08026 × 1016 fm | 2.10312 × 1016 fm | 2.12598 × 1016 fm | 2.14884 × 1016 fm | 2.1717 × 1016 fm | 2.19456 × 1016 fm | 2.21742 × 1016 fm | 2.24028 × 1016 fm | 2.26314 × 1016 fm | 2.286 × 1016 fm |
A span is the distance measured by a human hand, from the tip of the thumb to the tip of the little finger. In ancient times, a span was considered to be half a cubit. Sometimes the distinction is made between the great span (thumb to little finger) and little span (index finger to little finger).
| Name of unit | Symbol | Definition | Relation to SI units | Unit System |
|---|---|---|---|---|
| span | span | ≡ 9 in | = 0.2286 m | Imperial/US |
| spans | fermi | spans | fermi |
|---|---|---|---|
| 1 | = 2.286E+14 | 4 | = 9.144E+14 |
| 1.5 | = 3.429E+14 | 4.5 | = 1.0287E+15 |
| 2 | = 4.572E+14 | 5 | = 1.143E+15 |
| 2.5 | = 5.715E+14 | 5.5 | = 1.2573E+15 |
| 3 | = 6.858E+14 | 6 | = 1.3716E+15 |
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.
| Name of unit | Symbol | Definition | Relation to SI units | Unit System |
|---|---|---|---|---|
| fermi | fm | ≡ 1×10−15 m | ≡ 1×10−15 m | Metric system Non-SI |
| fermi | spans | fermi | spans |
|---|---|---|---|
| 1 | = 4.3744531933508E-15 | 4 | = 1.7497812773403E-14 |
| 1.5 | = 6.5616797900262E-15 | 4.5 | = 1.9685039370079E-14 |
| 2 | = 8.7489063867017E-15 | 5 | = 2.1872265966754E-14 |
| 2.5 | = 1.0936132983377E-14 | 5.5 | = 2.405949256343E-14 |
| 3 | = 1.3123359580052E-14 | 6 | = 2.6246719160105E-14 |
| spans | fermi |
|---|---|
| 1 | = 2.286 × 1014 |
| 4.3744531933508 × 10-15 | = 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 …) |