Apple Watch 42mm: two watches, different straps

If you’ve searched for “Apple Watch 42mm strap”, you’ve probably already noticed the problem: the results point in two different directions. It’s not because you’ve searched for the wrong thing. It’s because 42mm isn’t one size – it’s two.

Apple has used the same number for two completely different case sizes, across two different generations of the watch. A strap bought for the wrong “42mm” simply won’t fit. Here’s how to tell the difference in five minutes.

What is the 42mm paradox?

In short: 42mm meant “large” up to Series 9, and means “small” from Series 10 onwards.

• Older watch (Series 1, 2 or 3): 42mm was the larger of two sizes (the other was 38mm).
• Newer watch (Series 10 or 11): 42mm is the smaller of two sizes (the other is 46mm).

Two watches can therefore both be labelled “42mm” and still require completely different straps, as they belong to different size families with different strap widths.

The two strap families

The Apple Watch has used the same ‘slide-and-click’ clasp throughout its history, but the clasp comes in two widths. All watches within the same family share the same straps – regardless of the year or series they belong to.

Family 1 – the smaller cases

A strap bought for a Series 4 (40mm) will therefore fit perfectly on a Series 11 (42mm) – they belong to the same family, despite four different size names and a seven-year gap.

Family 2 – the larger cases

The same applies here: a 44mm strap from a Series 6 will fit an Ultra without any problems. The width is the same across the entire range; it’s just the case and the screen that have grown.

One exception: if you own an Ultra, we recommend a strap with reinforced links rather than a standard ‘Large’ strap from, for example, Series 3, particularly if the watch is used for diving or climbing. The width is the same, but the construction isn’t designed to withstand the same level of stress.

How to find out which model you have

If you’re unsure which generation your watch is, or can’t remember what you bought: turn the watch over. The size and series are engraved directly on the back of the case, near the heart rate sensors. This is the quickest and most reliable method – quicker than guessing based on the year of purchase.

How to choose the right strap

1. Read the engraving on the back of the case.
2. Find your model in the table above and note which family it belongs to.
3. Choose freely within that family – all straps in the same family will fit, regardless of which specific series they were originally designed for.

If you’d like to see our full range of Apple Watch accessories, regardless of size or generation, you’ll find them under Apple Watch straps. And if you already know which family your watch belongs to but are unsure how it compares with other brands, or want to understand the different attachment systems in more detail, we cover this in our compatibility guide.

Frequently asked questions

Will a strap bought for the Series 4 (40mm) fit a Series 11 (42mm)?

Yes. Both belong to the same strap family (Family 1), so the width of the attachment is identical even though the size names differ.

Can I use a 44mm strap on a Series 11 (42mm)?

No. The 44mm belongs to Family 2 and the 42mm (new) belongs to Family 1 – they have different strap widths and do not fit each other’s watches, regardless of how close the numbers are to each other.

How do I know what size my Apple Watch actually is?

Turn the watch over. The size and series are engraved on the back of the case, near the heart rate sensors.

Do straps for older Apple Watch models fit an Apple Watch Ultra?

In terms of measurements, yes – the Ultra belongs to Family 2, the same as Series 1–3 (the old 42mm models) and later. However, we recommend straps with reinforced links for the Ultra when diving, climbing or during other rugged use.