Measure the size of both straps jointly in millimeters (mm). You may also measure every one separately and include them up to get the full strap duration. Do not include the buckle as part of your strap duration measurement. We’re at this time tests new watch bands for this guide, https://allhealthmarket.com/product/aeahmarket/appleband/silicone-strap-for-apple-watch/strap-for-apple-watch-38-40-41-42-44-45-49mm-band-replacement-silicone-wristband-for-iwatch-8-7-6-5-4-3-2-1-se-ultra/