A watch case is the face of the watch, or the main area of the watch with the dial display and any other instruments. Most women's watches are approximately 26 to 36 mm (1 to 1.4 inches) in size, while most men's watches are 37 to 42 mm (1.4 to 1.9 inches) in size. There are a few methods for measuring the diameter of a wristwatch case.
Use a small cloth or paper tape measure, such as the ones used for sewing. You can buy these at most supermarkets, chemist shops and fabric suppliers. Make sure the tape measure includes millimetres on the measurement scale as inches aren't precise enough for this job.
Place the end of the tape measure (at "0") on one side of the watch face. If there are any stems on the side of the watch face, start the measurement at that point.
Pull the tape measure across the watch face at the widest width of the watch to the other side of the case, including any stems on the other side. Make sure the tape measure is taut across the watch face.
Write down the number of the case width. This is the diameter of the watch case. Note that when sellers or retailers refer to the "watch size," they are commonly referring to the thickness of the watch case or the length of the watch strap, not the diameter of the watch face.
Try using digital readers to measure the watch face. These tools are available at most jewellery shops or online, and use a small metal gauge to measure the watch face. The measurement is then displayed on a digital reader.