1. Use a tape measure to measure your wrist circumference. Measure around your wrist, above the bone, in the exact location where you would wear your watch. Make sure that the tape measure is not too tight or too loose. 2. If your wrist is 6 to 7 inches in circumference, you should typically go with 38mm, 40mm and 42mm watch cases. If your wrist circumference is 7 to 8 inches, you should be looking at 44mm to 46mm watch cases. If your wrist circumference is less than 6 inches then 34mm to 36mm watch cases are ideal.