You'll need to measure the lug width of your watch — that's the space between the two small posts (lugs) where the strap attaches to the watch case. Lay your watch flat, remove the old strap if possible, and use a ruler or caliper to measure that gap in millimeters. Common sizes are 18mm, 20mm, and 22mm. We also offer 16mm, 19mm, 21mm, 23mm, and 24mm widths in some straps. If you're not sure, just send us a message with your watch's make and model and we'll help you figure it out.