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.

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