The following information on watch sizes and the conversion tables is taken directly from the 1957 Watch Material Distributors Association of America Catalog (just laboriously re-typed into html format!). This is the most comprehensive set of tables and information I have seen on the subject. Although there are watch size gauges available from material suppliers, they don’t give exact numerical values and equivalents if you require precise conversion values. Hopefully this information will answer all of your watch size questions.

WATCH SIZES

A watch movement’s size is gauged by measuring it across its center on the dial side through its narrowest point. This system is used by the U.S. Customs in determining the amount dutiable on imported movements, and thus it has become associated with the sizing of all watch movements brought into this country. Although the use of the millimeter to determine watch sizes is growing steadily, the predominant system still in use is the LIGNE. The LIGNE is equal to the twelfth part of the French Inch, which is approximately 1/16th larger than our own inch. One French ligne=2.25583 or roughly 2¼ mm. Thus a 10 ligne movement should gauge 22.5mm. In inches this would measure .8881 as the LIGNE is equal to .0888 parts of an inch. (Note: These days, movement sizes are more commonly stated in millimeters – Rob B)

The SIZE as a gauge with which to measure movements is still used by some of the older American watch manufacturers to indicate the dimensions of their movements. The SIZE as a unit has its basis in the old “0” size movement. This movement measures 1 5/30″. A difference of 1/30″ accounts for each size smaller or larger than the starting point of the system, the “0” size watch movement. Thus a “1” size movement would be 1 6/30″, a “6” size movement 1 11/30″. Watch movements smaller than the “0” size are designated by adding zeros to the size. Thus a movement one size smaller than the “0” would be a 2/0 size movement and would gauge 1 4/30″, the 3/0 size would then measure 1 3/30. Conversely, the “25” size watch movement would measure just 2 full inches.

A good way to remember sizes of the American and Swiss gauges is to associate the unit we know best, the inch, with both the American and Swiss equivalents. For example, the 6/0 size American movement is exactly one inch in diameter (“0″ size = 1 5/30″ – 5/30” = one inch). Also, the Swiss 11¼ Ligne is just a few ten thousandths of an inch under one inch, and can be called a one-inch watch movement.

Original text copyright © 2000 by Rob Berkavicius (Rob B)
September 6th 2000, Perth, Western Australia.