Can you use a 44mm strap on a 40mm and vice versa?

The 38 and 40 use the same watch band

The 42 and 44 use the same larger watch band

While you can stick a 44mm watch band on a 40mm watch, the mounting tabs will extend from the side of the 40mm case and can snag on things and will not look nice.

yes

https://www.apple.com/shop/product/MWUK2AM/A/40mm-pomegranate-sport-band-s-m-m-l?fnode=4fb108d02cda394c02898d2ae257173988e4bdc238931ff2938d0938eead986cb39458ad1ed4fb08e055bb9fe7501afa7b23942b1e3f227a7c25a14f48ba54c4cf8a964b981e88716e954f99b4bee428

Case Size

• 40mm
• 44mm

Works with all versions of Apple Watch.

Learn more about band compatibility