Convert between the American size system, millimeters, and ligne. The American system measures the movement plate diameter using the Lancashire gauge formula.
Type in any field — the others update automatically.
Case is larger than the movement. These are approximate ranges — actual case size depends on case maker and style.
Standard sizes with the Lancashire gauge formula: Diameter (inches) = (Size + 5) / 30
| Size | Plate (mm) | Plate (ligne) | Plate (inches) | Typical Case (mm) | Notes |
|---|---|---|---|---|---|
| 18s | 44.86 | 19.89 | 1.767" | 55–58 | Largest common. Early railroad. |
| 16s | 43.18 | 19.14 | 1.700" | 50–53 | Standard railroad (post-1900) |
| 14s | 41.50 | 18.40 | 1.633" | 48–50 | Less common transitional size |
| 12s | 39.82 | 17.65 | 1.567" | 45–48 | Gentleman's dress watch |
| 10s | 38.14 | 16.91 | 1.500" | 42–45 | Smaller dress watch |
| 8s | 36.46 | 16.16 | 1.433" | 40–42 | Lady's/small gentleman's |
| 6s | 34.78 | 15.42 | 1.367" | 38–40 | Lady's larger size |
| 4s | 33.10 | 14.67 | 1.300" | 36–38 | Lady's watch |
| 2s | 31.42 | 13.93 | 1.233" | 34–36 | Lady's watch |
| 0s | 29.74 | 13.18 | 1.167" | 33–35 | Lady's common size |
The American pocket watch size system uses the Lancashire gauge, developed in England and adopted by American manufacturers. It measures the movement plate diameter (pillar plate), not the case.
Formula: Diameter (inches) = (Size + 5) ÷ 30
So a "16-size" movement has a plate diameter of (16 + 5) / 30 = 21/30 inch = 0.700" × 2 ... wait. The actual formula gives the diameter as (Size + 5)/30 inches. For 16s: 21/30" = 0.700" ... but the actual standard plate diameter of a 16s is 1.700" (43.18mm).
Corrected formula: Diameter (inches) = (Size + 5) / 30 + 1.000"
The case is always larger than the movement — typically 8–14mm greater in diameter depending on case thickness and style. When buying a case or fitting a movement, always measure the actual movement and case opening rather than relying on size number alone.