How to size a ring
The most accurate way is to measure the inside diameter of an existing ring with a calliper. Lay the ring flat and measure straight across the inside — this is the ISO millimetre value, which converts to every regional system.
If you only have a finger to measure, wrap a strip of paper snugly around the base of the finger, mark where it overlaps, and measure the strip with a ruler. That length is the circumference; divide by π (3.14) to get the inner diameter.
Common sizes
- US 6 ≈ UK L 1/2 ≈ EU 51.5 ≈ JP 12 (16.5 mm) — average women's ring finger.
- US 7 ≈ UK N 1/2 ≈ EU 54 ≈ JP 14 (17.3 mm) — common ring-finger size.
- US 9 ≈ UK R 1/2 ≈ EU 59 ≈ JP 18 (19.0 mm) — average men's ring finger.
- US 10 ≈ UK T 1/2 ≈ EU 61.6 ≈ JP 20 (19.8 mm).
Tips
Measure your finger at the end of the day, when it is at its largest. Cold fingers can be a half-size smaller than warm. If you are between two sizes, choose the larger — a ring that is too tight is much harder to remove than one slightly loose.
FAQ
Why does the UK use letters? Historically British jewellers used a letter sequence with half-sizes (L 1/2, M 1/2). The letters jump in roughly 0.4 mm steps.
Are wedding band sizes different? Wider bands fit tighter than thin bands at the same numerical size — it is common to size up by 1/4 to 1/2 for a 6 mm or wider band.