Here's a simple guide to figuring out your ideal watch size with just a ruler
Step 1: Use a ruler to measure the top to bottom of your wrist in millimetres (example above 49mm top to bottom)
Step 2: Take your wrist size and now minus 10mm to get a range of watches that will fit. (example 49mm - 10mm = 39mm watch size)
* We use 10mm as an example to minus because most watch lugs can range from a minimum of 2mm all the way to 10mm of added length from top to bottom.
Step 3: Find watches in this range that you calculated and add or minus additional mm. (example Look for watches from 37mm, 38mm, 39mm, 40mm, 41mm)
Step 4: When possible, try on watches in these sizes to get an idea of how it fits on your wrist.
Use this method as a guideline for finding your ideal watch size and what you prefer on your wrist. Even 2 watches in the same mm size can vary differently on the wrist.