How to find your apple watch band size: 38mm or 40mm? 42mm or 44mm?
Each series of the Apple watch comes in two different sizes depending on the series you purchased.
When looking for the suitable apple watch band size, the 38mm or 40mm is the same adaptor slot regardless of series. Likewise with the 42mm or 44mm. To put it simply, the 38mm (Series 1, 2 or 3) is the same adaptor size as the newer series 40mm (Series 4 or 5); and the 42mm (Series 1, 2 or 3) is the same adaptor size as the 44m (Series 4 or 5).
When selecting the correct watch band size, refer to the back of your watch for correct sizing: