The Apple Watch is available in 4 different sizes: 38, 40, 42, and 44 millimeters. The straps for the Apple Watch 38 and 40 millimeters fit on both watches. If you have the Apple Watch 38mm and you want to buy a 40-millimeter strap, you can. The watch strap connector still fits the Apple Watch 38mm. This also applies the other way around. You can also wear Apple Watch 38mm straps on the Apple Watch 40mm. You can choose from a lot of watch straps for your Apple Watch.