The size of a watch movement

Watchmakers Handbook

As a matter of interesting information relating to the description of American watch movements, as being of certain designated “sizes”, let us say that this system of measurement is identical with that established and used by English watchmakers, and relates to the extreme diameter of the pillar plate. Being English, its unit of measurement is the English inch, and the zero, or starting point, is one inch plus five-thirtieths of an inch (1⁵/₃₀), which is the diameter of an 0 size movement. Each ¹/₃₀ an inch added to this measurement represents the size of a movement. Therefore, 1⁵/₃₀ plus ¹/₃₀ will be the diameter of a one size movement: a six size should measure 1¹¹/₃₀, a twelve size 1¹⁷/₃₀, an eighteen size 1²³/₃₀, etc.

NOTE: In order to express this in millimeters we must multiply by 25.4, which is the metric equivalent of an inch, so the general formula will be (³⁵/₃₀ + ^SIZE/₃₀) X 25.4: and the formula for 12 size, (³⁵/₃₀ + ¹²/₃₀) X 25.4 = 39.8 millimeters.

The Ligne was originally accepted abroad as the unit of outside measurement on watch movements.

Divided into 12/12 (douziemes) this sub-division figures as unit for thickness of plates. 1 ligne = 12/12 = 2.256 millimeters.

Most manufacturers have abandoned this obsolete method of measurement, replacing it advantageously with the metric system.

Measurements relating to watch movements

This table gives imperial and metric equivalents of the
traditional English/American system.

Size	Thirtieths	Inches	mm	lignes
0	35	1.17	29.63	13.26
1	36	1.20	30.48	13.64
2	37	1.23	31.33	14.02
3	38	1.27	32.17	14.39
4	39	1.30	33.02	14.77
5	40	1.33	33.87	15.15
6	41	1.37	34.71	15.53
7	42	1.40	35.56	15.91
8	43	1.43	36.41	16.29
9	44	1.47	37.25	16.67
10	45	1.50	38.10	17.05
11	46	1.53	38.95	17.45
12	47	1.57	39.79	17.80
13	48	1.60	40.64	18.18
14	49	1.63	41.49	18.56
15	50	1.67	42.33	18.94
16	51	1.70	43.18	19.32
17	52	1.73	44.03	19.70
18	53	1.77	44.87	20.08
19	54	1.80	45.72	20.45
20	55	1.83	46.57	20.83
21	56	1.87	47.41	21.21
22	57	1.90	48.26	21.59
23	58	1.93	49.11	21.97
24	59	1.97	49.95	22.35