Apple has sold cases in eight sizes across its watch line. For band purposes there are two groups, and bands move freely within each one. There is one exception that catches people out constantly.
The two band sizes
Smaller group: 38mm, 40mm and 41mm cases, plus the 42mm case on Series 10, 11 and 12.
Larger group: 44mm, 45mm, 46mm and the 49mm Apple Watch Ultra, plus the 42mm case on Series 3 and earlier.
Any band made for one case in a group fits every other case in that group, regardless of series or year.
The 42mm trap
Read those two lists again and you will spot it: 42mm appears in both.
Apple used 42mm twice, a decade apart, on watches with different band connectors. The original 42mm from Series 3 and earlier takes the larger band. The 42mm reintroduced on Series 10 takes the smaller one.
So "42mm" on its own does not tell you which band you need. You need the series as well.
If you are shopping for a 42mm Apple Watch, check the back of the case or open Settings, General, About to confirm which series you have before ordering.
Which case do I have?
On the watch, open Settings, then General, then About. The model name and case size are listed there.
The case size is also engraved on the back of the watch, though it is small and easier to read in good light.
If you are buying a second band, the simplest check is the one you already own: if it fits, the new band in that size will too.
A note on cases and bundled cases
Protective cases, and bands that ship with a case built in, are matched to a specific model and case size rather than to the two-group system. For those, follow the product's own size options rather than the general band guide.
What the size does not tell you
Whether it will look right. A band that fits a 49mm Ultra also fits a 44mm Series watch, but a wide strap sized for the Ultra can overwhelm the smaller case, and a delicate mesh can look lost on an Ultra.
As a rough rule: slimmer bands suit the small group, more substantial bands suit the large group.
The exception: stretch bands
Solo loops and similar stretch bands have no clasp and no adjustment, so they use their own sizing scale on top of the case size. A solo loop is sized to your wrist, not just your watch.
Get that wrong by one size and it either slides, which ruins heart-rate readings, or leaves a mark. Measure your wrist properly rather than estimating from the hole you normally use.
Changing a band
Hold the watch face down, press and hold the band release button on the back of the case, then slide the band sideways out of its channel. Slide the new one in from the same side until it clicks.
The two halves are different lengths, so note which end was which before you set the old band down.
Getting the fit right
The watch should sit snug against the skin about a finger's width above the wrist bone. Apple's guidance for workouts is to wear it slightly tighter and a little higher up the arm, then loosen it afterwards.
Too loose and the optical sensor reads motion rather than pulse. Too tight restricts blood flow, which skews readings the other way.
Shop Apple Watch bands.
