# Apple Watch - Size and compatibility

**Finding Your Size**

![Apple Watch reading the size](https://cdn.shopify.com/s/files/1/0566/5747/7714/files/Apple_watch_find_size.png?v=1761793242)

No guessing, no charts — just read the case size in millimeters on the back.

**Step 1 — Confirm the mm size**

Gently wipe the case back and look for **mm**. Examples: **41mm**, **45mm**, **49mm**.

**Step 2 — Pick your size**

Small • 38/40/41 mm

Large • 42/44/45/49 mm

Pick the matching variant on the product page for a perfect click-in fit.

Tip: In a pinch, the Watch app → About → Model also confirms your case size.

**Band Compatibility**

Apple’s band system is designed to be stable across generations. That’s why an older **38mm** band still fits a new **41mm** case, and why a **44mm** band locks perfectly into a **45mm** or **49mm Ultra**. Know your size family once; buy with confidence forever.

**Works with:**  
  
•**Small Family (38/40/41 mm)**  
→ Series 1–9 (small), SE  
  
•**Large Family (42/44/45/49 mm)**  
→ Series 1–9 (large), SE (large), Ultra & Ultra 2 (49mm)

Your watch’s _name_ may change when you upgrade — the _connector family_ doesn’t.

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> Source: [Nothing but Bands](https://nothingbutbands.com/pages/apple-watch-size-and-compatibility)