Pocket Watch Sizes

You will notice that an American pocket watch size is
indicated by a single number (18s), or a number
followed by a -0 (3-0).  These numbers are not
arbitrary, but are numbers that the watch exceeds 1
5/30” of an inch in increments of thirtieths of an inch.

For example, an 18s watch is 1 5/30” plus 18/30”,
total measurement equaling 1 23/30”.  This
measurement was of the diameter of the movement
plate behind the dial, not the case.  A century ago, it
was common to select the movement at the jeweler,
and then select a case separately (the case was
often made by a different manufacturer), so a
standard for watch sizes was crucial.

A '0' size watch would be 1 5/30”, and a 2-0 (00)
watch would be 1 5/30” minus 1/30” (1 4/30”) and so

Common 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

European watches were commonly sized in
Millimeters, or lignes (pronounced “lines”).  A ligne is
2.26 mm.