Which Apple Watch do you have?
You can easily see which Apple Watch you have, because it is engraved on the back of the Watch. This shows which generation of Apple Watch you have, and what size case you have. This size is indicated in millimeters, for example: 40mm or 42mm etc.
Small sizes: 38mm, 40mm, and 41mm
Do you have the new apple watch 10? small size 42mm,
then choose for the 38/40/41 mm
Large sizes: 42mm, 44mm, 45mm, and 49mm
Do you have the new apple 10? Big mate 46mm,
then choose for the 42/44/45/49 mm
So take a good look at the back of your Apple Watch to see what size it is.
