I have bought 3 different Glass screen protectors; both 37 & 38 mm. They both looked great (the 37 mm with a little room within the black bezel), but none of them lasted more than 3 days before they were striated around the edges or strait-up cracked, often in multiple places. Too brittle, and I guess I'm a bit rough- playing with the dogs and tennis never seemed that rough before... I found "[ 3-Pcs] Universal Round Smart Watch HD Screen Protector, Anti-Scratch, Anti-Fingerprint, Bubble Free (37mm)" on Amazon Ultra Clear PET round Watch Screen Protector. So far, no problems. You can select 37 or 38 mm.