keepcalculator

06 Numbers and Powers

Apple Watch Series 4 40mm & 44mm Band Compatibility

Card No. 06·6564 · Updated · Standard formula, shown below

Yes, the Apple Watch Series 4 40mm model is compatible with 38mm bands, and the 44mm model is compatible with 42mm bands. Apple designed the Series 4 with the same band attachment mechanism and case width at the connection points as the earlier 38mm and 42mm models, so all bands from the original Apple Watch through Series 3 fit the Series 4 perfectly.

Background: Why the Sizes Changed but Bands Didn’t

When Apple released the Series 4 in September 2018, it introduced new case sizes: 40mm and 44mm, replacing the 38mm and 42mm sizes used since the first Apple Watch in 2015. Despite the larger numbers, the actual width of the case where the band attaches remained the same. The 40mm case is the same width at the connection as the 38mm, and the 44mm matches the 42mm. This was a deliberate design choice to preserve compatibility with the existing band ecosystem. The increased size came from a larger display and thinner bezels, not from widening the band slots.

How the Band Attachment Works

Apple Watch bands slide into a recessed slot on the top and bottom edges of the case. A small button on the back of the case releases the band. The slot dimensions are identical across the compatible sizes. Here’s what you need to know:

  • The 40mm and 38mm cases use the same band slot width (about 22mm).
  • The 44mm and 42mm cases use the same band slot width (about 24mm).
  • All bands with the same size designation (38/40mm or 42/44mm) are interchangeable.
  • Bands are not cross-compatible between the two size groups—a 38/40mm band will not fit a 42/44mm case.

How to Choose the Right Band Size

If you’re buying a new band, always check the size compatibility listed on the product. Most third-party bands specify “38/40mm” or “42/44mm” on the packaging. Here’s a quick guide:

  • If you own a 40mm or 38mm Apple Watch, choose bands labeled 38mm or 40mm.
  • If you own a 44mm or 42mm Apple Watch, choose bands labeled 42mm or 44mm.
  • When in doubt, measure the width of your current band’s connector—it should be about 22mm for 38/40mm and 24mm for 42/44mm.

Common Problems and Misconceptions

One common misconception is that the Series 4’s larger display means the case is physically wider at the band connection. It isn’t. The case is slightly taller and wider overall, but the band attachment area is unchanged. Another issue: some third-party bands made specifically for the Series 4 might have a slightly tighter fit due to manufacturing tolerances, but this is not a size issue. If a band feels loose or wiggles, it’s likely a defect in the band, not a compatibility problem. Also, note that the Series 4’s speaker grille moved to the left side, but this doesn’t affect band attachment.

Comparison with Later Models

The compatibility extends beyond the Series 4. The Series 5, 6, SE, and Series 7 (41mm and 45mm) also maintain the same band slot widths. Here’s a quick breakdown:

  • Series 4, 5, 6, SE (40mm and 44mm) use the same bands as 38mm and 42mm models.
  • Series 7, 8, 9, and 10 (41mm and 45mm) also use the same bands as 38/40mm and 42/44mm respectively.
  • The Apple Watch Ultra (49mm) uses a different, larger band connector (about 25mm) and is not compatible with standard bands.

Cost and Availability of Bands

Because the 38/40mm and 42/44mm sizes have been standard for many years, there is a huge aftermarket selection. You can find bands at a wide range of prices, from budget third-party options to premium Apple bands. The compatibility also means you can buy older bands designed for the original Apple Watch and use them on a Series 4 without any issue. This is a cost-saving advantage for those who already own bands from previous models.

Frequently Asked Questions

Will a 38mm band fit a 40mm Apple Watch?

Yes, a 38mm band fits a 40mm Apple Watch. The 40mm case uses the same band connector width as the 38mm case, so all bands labeled 38mm or 40mm are interchangeable.

Will a 42mm band fit a 44mm Apple Watch?

Yes, a 42mm band fits a 44mm Apple Watch. The 44mm case uses the same band connector width as the 42mm case.

Can I use a 44mm band on a 40mm watch?

No, a 44mm band will not fit a 40mm watch. The connector width is different (24mm vs 22mm), so the band won’t slide in properly.

Does the Series 4 come with a band in the box?

Yes, the Series 4 ships with a band in the box, typically a sport band or sport loop. You can choose the size (38/40mm or 42/44mm) when purchasing.

Are all Apple Watch bands the same size?

No, there are two main sizes: 38/40mm and 42/44mm. The Apple Watch Ultra uses a third, larger size. Always match the band size to your watch case size.

Related

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.

More cards in this drawer

The whole Numbers and Powers drawer →