Watch Sizes

Watch Size Fractional Inch Decimal Inch Lignes Millimeters
18 1 23/30 1.766 19.87 44.86
17 1 22/30 1.733 19.50 44.02
16 1 21/30 1.700 19.12 43.18
15 1 20/30 1.666 18.75 42.32
14 1 19/30 1.633 18.37 41.48
13 1 18/30 1.600 18.00 40.64
12 1 17/30 1.566 17.62 39.78
11 1 16/30 1.533 17.25 38.94
10 1 15/30 1.500 16.87 38.10
9 1 14/30 1.466 16.50 37.24
8 1 13/30 1.433 16.12 36.40
7 1 12/30 1.400 15.75 35.56
6 1 11/30 1.366 15.37 34.70
5 1 10/30 1.333 15.00 33.86
4 1 9/30 1.300 14.62 33.02
3 1 8/30 1.266 14.25 32.16
2 1 7/30 1.233 13.87 31.32
1 1 6/30 1.200 13.50 30.48
0 1 5/30 1.166 13.12 29.62
2/0 1 4/30 1.133 12.75 28.78
3/0 1 3/30 1.100 12.37 27.94
4/0 1 2/30 1.066 12.00 27.08
5/0 1 1/30 1.033 11.62 26.24
6/0 1 0/30 1.000 11.25 25.40
7/0 29/30 .966 10.87 24.54
8/0 28/30 .933 10.50 23.70
9/0 27/30 .900 10.12 22.86
10/0 26/30 .866 9.75 22.00
11/0 25/30 .833 9.37 21.16
12/0 24/30 .800 9.00 20.32
13/0 23/30 .766 8.62 19.46
14/0 22/30 .733 8.25 18.62
15/0 21/30 .700 7.87 17.78
16/0 20/30 .666 7.50 16.92
17/0 19/30 .633 7.12 16.08
18/0 18/30 .600 6.75 15.24
19/0 17/30 .566 6.37 14.39
20/0 16/30 .533 6.00 13.54
21/0 15/30 .500 5.62 12.69
22/0 14/30 .466 5.25 11.85
23/0 13/30 .433 4.87 11.00
24/0 12/30 .400 4.50 10.15
25/0 11/30 .366 4.12 9.31
26/0 10/30 .333 3.75 8.46

Watch movements are measured using the Lancashire system or Lancashire Gauge. In this system, 1 5/30th inches is the "base" measurement and is called the 0-size. Each 1/30th of an inch adds 1 (or subtracts 1) to this base. A watch is measured through the center of the lower plate of the movemen on the dial side. If the movement is asymmetrica it is measured at its narrowest point. Note that the case and the dial are not included in this measurement, only the movement. Measuring the dial is usually a fair approximation of the lower plate measurement.