Early pocket watches movements and cases were purchased separately at a jeweler. A size standard was needed, so a somewhat convoluted number system using 1/30″ was used (for reasons unknown to me). For simplicity’s sake, I have created a mm conversion chart below.
Standard Sizes and MM equivalent
20s 46.56 mm
18s 44.86 mm
16s 43.18 mm
14s 41.48 mm
12s 39.78 mm
10s 38.10 mm
8s 36.40 mm
6s 34.70 mm
0s 29.62 mm
3/0s 27.94 mm
The ligne (pronounced line) is used for European watches. A ligne is 2.26 mm.
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