To get the most precise pocket watch measurement, you will want to measure the movement’s lower plate below the dial. Since the dial often extends beyond the movement, it’s not an accurate way of measuring the size of your pocket watch, however, with a well-trained eye it might be close enough to guess.
Movement vs Case Size
It’s important to know that a pocket watch size has nothing to do with the outside case diameter. A pocket watch can have various case sizes depending on the style and design. The case diameter is good for knowing how big the pocket watch is overall, if it will fit in your vest or watch pocket for example, but it is not relevant to the chart above.
Common Pocket Watch Sizes
Pocket watches come in all different sizes, but the most common will be 18s, 16s and 12s for men, or 0s, 6s and 8s for ladies pocket watches. 10s or 12s can be considered unisex. Early on, railroad pocket watches were allowed to be 18s or 16s, but eventually were required to be 16s.
What Does the S Mean?
The S next to the number just means “Size”. For example 18 is the same as 18S, and also the same as 18-Size. This S is usually added to the number so that you know it’s a size number rather than a jewel count. Similarly, you may also see something like 7J for jewels. Some dealers and enthusiasts might type these sizes differently.
Pocket Watch Sizes
These measurements are not related to the size of the case or dial. This chart converts American pocket watch sizes from inches to millimeters. You will notice from the chart below that pocket watches are typically measured in 30ths of an inch, also known as the “Lancashire Gauge” adopted from England. Remember, the sizes below refer to the movement size.