Pocket watches are often described as 18-size, 16-size, or something like that. What does that mean exactly? Like any good imperial unit, it’s complicated.
Note: One important thing to remember is that watch size is based on the size of the movement, not the watch case. This can be deceiving, as watch cases vary a lot.
Most American watches are sized on the Lancashire scale. The Lancashire scale is based on a 0-sized watch being 1″, but to account for the “fall” – the the extra amount added to the bottom plate of the watch to keep the watch from falling out of the case – you need to add 5/30″, so a 0-sized watch on the Lancashire scale would be 1 5/30″, or 1.667″. Each step larger than this adds 1/30″, therefore a 12-size watch = 1″ + 5/30″ + 12/30″, or 1.567″ and an 18-size watch = 1″ + 5/30″ + 18/30″, or 1.767′.
The other common scale is the Swiss Ligne scale, with “Ligne” being pronounced “line”. This scale is much more reasonable – one “Ligne” = 2.256mm or 0.089″. There are 12 Ligne per French inch.
Check out the table below for an exhaustive comparison. Happy measuring!
| Lignes (Swiss watches) | Lancashire (American watches) | Millimeters | Inches |
|---|---|---|---|
| 3 3/4 | 26/0 | 8.459 | 0.3330 |
| 4 | 9.023 | 0.3552 | |
| 4 1/4 | 25/0 | 9.587 | 0.3774 |
| 4 1/2 | 24/0 | 10.151 | 0.3996 |
| 4 3/4 | 10.715 | 0.4219 | |
| 5 | 23/0 | 11.279 | 0.4441 |
| 5 1/4 | 22/0 | 11.843 | 0.4663 |
| 5 1/2 | 12.407 | 0.4885 | |
| 5 3/4 | 21/0 | 12.971 | 0.5107 |
| 6 | 20/0 | 13.535 | 0.5329 |
| 6 1/4 | 14.099 | 0.5551 | |
| 6 1/2 | 19/0 | 14.663 | 0.5773 |
| 6 3/4 | 18/0 | 15.227 | 0.5995 |
| 7 | 15.791 | 0.6217 | |
| 7 1/4 | 17/0 | 16.355 | 0.6439 |
| 7 1/2 | 16/0 | 16.919 | 0.6661 |
| 7 3/4 | 17.483 | 0.6883 | |
| 8 | 15/0 | 18.047 | 0.7105 |
| 8 1/4 | 14/0 | 18.611 | 0.7327 |
| 8 1/2 | 19.175 | 0.7549 | |
| 8 3/4 | 13/0 | 19.739 | 0.7771 |
| 9 | 12/0 | 20.303 | 0.7993 |
| 9 1/4 | 20.867 | 0.8215 | |
| 9 1/2 | 11/0 | 21.430 | 0.8437 |
| 9 3/4 | 10/0 | 21.994 | 0.8659 |
| 10 | 9/0 | 22.558 | 0.8881 |
| 10 1/4 | 23.122 | 0.9103 | |
| 10 1/2 | 8/0 | 23.686 | 0.9325 |
| 10 3/4 | 24.250 | 0.9547 | |
| 11 | 7/0 | 24.814 | 0.9769 |
| 11 1/4 | 6/0 | 25.387 | 0.9995 |
| 11 1/2 | 25.942 | 1.0213 | |
| 11 3/4 | 5/0 | 26.506 | 1.0435 |
| 12 | 4/0 | 27.070 | 1.0657 |
| 12 1/2 | 3/0 | 28.198 | 1.1102 |
| 12 3/4 | 2/0 | 28.762 | 1.1324 |
| 13 | 0 | 29.326 | 1.1546 |
| 13 1/2 | 1 | 30.453 | 1.1989 |
| 14 | 2 | 31.581 | 1.2433 |
| 14 1/4 | 3 | 32.046 | 1.2617 |
| 14 3/4 | 4 | 33.273 | 1.3100 |
| 15 | 5 | 33.838 | 1.3322 |
| 15 1/2 | 6 | 34.955 | 1.3762 |
| 15 3/4 | 7 | 35.529 | 1.3988 |
| 16 | 36.093 | 1.4210 | |
| 16 1/4 | 8 | 36.657 | 1.4432 |
| 16 1/2 | 9 | 37.221 | 1.4654 |
| 17 | 10 | 38.349 | 1.5098 |
| 17 1/4 | 11 | 38.913 | 1.5320 |
| 17 3/4 | 12 | 40.041 | 1.5764 |
| 18 | 13 | 40.605 | 1.5986 |
| 18 1/2 | 14 | 41.733 | 1.6430 |
| 18 3/4 | 15 | 42.297 | 1.6652 |
| 19 | 42.861 | 1.6874 | |
| 19 1/4 | 16 | 43.425 | 1.7096 |
| 19 1/2 | 17 | 43.989 | 1.7319 |
| 20 | 18 | 45.117 | 1.7763 |
| 21 | 21 | 47.372 | 1.8650 |
| 22 | 23 | 49.628 | 1.9539 |
| 23 | 26 | 51.884 | 2.0427 |
| 24 | 29 | 54.140 | 2.1315 |
| 25 | 32 | 56.396 | 2.2203 |
| 26 | 34 | 58.652 | 2.3091 |
| 27 | 37 | 60.907 | 2.3979 |
| 28 | 40 | 63.163 | 2.4867 |
| 29 | 43 | 65.419 | 2.5756 |
| 30 | 45 | 67.675 | 2.6644 |