Apple Watch Series 4, Series 5, and Series 6 come in 40mm and 44mm case sizes, so they're the same. As long as you get the same size for the new Apple Watch, the existing bands should fit seamlessly. Unfortunately, Apple Watch bands aren't universal. You can try on an Apple Watch band if you have an Apple store nearby, but if you don't, you need to measure a little more so Apple doesn't send the wrong size.
If you have an older Apple Watch Series 0, 1, 2 or 3, your bands should be for the 38mm or 42mm case. Made of leather, the modern buckle is the only Apple band that is exclusive to the smaller Apple Watch models of 40 and 41 mm. These can be useful when measuring the circumference of the wrist to calculate the size of the Apple Watch band. While it may seem at first that Apple has unceremoniously eradicated support for all previous models, don't worry, it didn't.
However, the company made it a little more confusing to quickly find the accessory made for your watch size. The company has promised that all existing Apple Watch bands will work with the Series 7 and its extended displays, so if you've accumulated a collection of great bands, it won't be useless if you decide to upgrade it. The modern buckle, for example, is limited to the 40 and 41 mm models, but for the most part Apple Watch straps fit a wide range of wrist sizes. There are some Apple Watch bands that need a specific size and others that aren't designed to fit every wrist.
Both bands have an elastic quality that allows them to slide over one hand and contract at the wrist, and these bands stretch over time, so it may be worthwhile to reduce the size of the measurement. The Modern Buckle is another strap that focuses more on fashion than functionality, and it's not designed to get wet. However, if you use third-party Apple Watch bands, the band may wear out over time. In case you didn't know, they're only compatible with Apple Watch SE, Series 4, and newer models.
Sizing is especially complicated with the Solo Loop and Solo Braided Loop, so the measurements of these watches must be accurate. Apple's printed guide provides a comparison of the size of the Apple Watch on the wrists for a powerful visual reference.
