At the bottom of this article you will find a link that shows a detailed chart of watch sizes.

The Lancashire gauge for determining watch sizes is of English origin and is the standard commonly used by American watch manufacturers. By this system 1 5/30 inches was taken as a basing figure and was called naught (0) size. Every 1/30 of an inch added increased the size one number, every subtraction of 1/30 of an inch decreased the size one number. The size of a watch is determined by its width through the center at the narrowest part of the dial side of the lower plate. The table below shows Lancashire gauge watch sizes in terms of fractions of inches, decimal inches, lignes and millimeters.

You have all heard that a watch is called a 16 size, 18 size or even a 6/0 size. But what does this really mean in terms that we can understand? There are two ways that watches are measured to determine its size. There is the American movement sizes based on the Lancashire Gauge. The chart below uses that system as it is the most common one in use.

The next system was the Ligne system which was a Swiss system based on the decimal system. Most wrist watches are measured using this millimeter and decimal system. For example a 10 Ligne watch would be a gents size wrist watch and a 5 Ligne watch would be a small ladies size wrist watch.

Now where do you measure from on the watch. For an accurate measurement, the movement should be removed from the watch case. People that deal with watches daily can look at them and know the size without measuring.

Now that the movement has been removed from the watch case, you simply measure the diameter of the main pillar plate of the watch. The pillar plate holds the dial. Care must be taken to NOT measure the dial, but make your measurement just below the dial. Now that you have the measurement either in Inches or Millimeters you can observe the chart at Antique Time and determine the size of your watch.

The number under size will be a number like 20, 18, 16, etc. Next you will have the size in inches. This is a 1 followed by a fraction such as 1 25/30. Next you will have the Millimeter size of the watch. This is a number such as 46.56.

Comparative values of standards of measurement.  1 inch equals 25.4 millimeters, 1 millimeter equals .03937 inches, and 1 ligne equals 2.25 millimeters.

Detailed Chart of Watch Sizes.