Above answer is incorrect. Probably because they aren’t familiar w/the Series 5 watches. A band that fits 38 also fits 40mm. It says right on the box 38/40mm. I just got mine for the Series 5. Fits the watch case perfectly, but is a bit snug on my wrist. I have a bigger wrist than many women and I like to wear the bands up a little higher than some people though.