16.5 mm ring size
A 16.5 mm inside diameter is about US 6 — UK L, EU 52, Japan 12 (51.9 mm circumference). For an exact fit, order by inside circumference in mm.
Diameter & circumference are exact (ISO 8653). UK letters and Japanese 号 numbers vary up to about half a size between jewelers — when ordering, the inside circumference in mm is the definitive measure.
How ring sizing systems relate
The US/Canada use numbers, the UK/Australia use letters, and the EU/ISO system is simply the ring's inside circumference in millimetres. Japan uses its own 号 (gō) numbers. Because the mm measurement is the physical truth, it's the most reliable way to order across countries.
Ring size conversion chart (US · UK · EU · Japan · mm)
| US | UK/AU | EU | Japan | Diameter | Circumference |
|---|---|---|---|---|---|
| 3 | F | 44 | 4 | 14.07 mm | 44.2 mm |
| 4 | H | 47 | 7 | 14.88 mm | 46.8 mm |
| 5 | J | 49 | 9 | 15.69 mm | 49.3 mm |
| 6 | L | 52 | 12 | 16.51 mm | 51.9 mm |
| 7 | N | 54 | 14 | 17.32 mm | 54.4 mm |
| 8 | P | 57 | 16 | 18.13 mm | 57 mm |
| 9 | R | 60 | 18 | 18.95 mm | 59.5 mm |
| 10 | T | 62 | 21 | 19.76 mm | 62.1 mm |
| 11 | V | 65 | 23 | 20.57 mm | 64.6 mm |
| 12 | Y | 67 | 25 | 21.38 mm | 67.2 mm |
| 13 | Z+1 | 70 | 27 | 22.2 mm | 69.7 mm |
Diameter = 11.63 + 0.8128 × US size; circumference = π × diameter (ISO 8653). UK/Japan columns follow published charts and can differ ±½ size by jeweler.
How to measure your ring size at home
- Wrap a strip of paper or string around the base of your finger.
- Mark where it overlaps and measure the length in mm — that's your inside circumference.
- Find that circumference in the chart above (or divide by π for the diameter).
- Measure at the end of the day when fingers are largest, and size up if between sizes.
FAQ
US 7 is about UK N, EU 54, and Japanese size 14, with an inside diameter of 17.3 mm and circumference of 54.4 mm. UK and Japan figures vary slightly by jeweler — the mm value is exact.
Not perfectly — different jewelers map US numbers to UK letters with up to ½-size differences. If precision matters (engagement rings!), order by inside circumference in mm.
If you have the inside circumference, that's essentially the EU/ISO size. For the inside diameter, multiply by π to get circumference, then read the chart. This tool does it for you.
Go with the larger size, especially for wider bands (they fit tighter), and measure when your hands are warm. A slightly loose ring is safer than one that won't come off.