Ring sizes are different in almost every country. The international standard (ISO 8653) simply uses the inside circumference in millimetres, which France also uses, while Germany uses the inside diameter. The US uses numbers in quarter steps and the UK uses letters with half sizes. Measure at the end of the day when fingers are largest, and remember wide bands fit tighter.
How to use the Ring Size Converter
- I measured
- Measurement
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose whether you measured the inside diameter of a ring or the circumference of your finger (or a strip of paper wrapped around it), and enter the measurement.
Ring size chart
| US | Diameter (mm) | Circumference (mm) | UK |
|---|---|---|---|
| 4 | 14.9 | 46.8 | H |
| 5 | 15.7 | 49.3 | J |
| 6 | 16.5 | 51.9 | L |
| 7 | 17.3 | 54.4 | N½ |
| 8 | 18.1 | 57.0 | P½ |
| 9 | 18.9 | 59.5 | R½ |
| 10 | 19.8 | 62.1 | T½ |
| 11 | 20.6 | 64.6 | V½ |
| 12 | 21.4 | 67.2 | X½ |
| 13 | 22.2 | 69.7 | Z½ |
Inputs explained
| Field | What to enter |
|---|---|
| I measured | Choose one: Inside diameter of a ring, Inside circumference / finger |
| Measurement | Units: mm, in, cm |
What each result means
- Inside circumference = pi×[Inside diameter]
- US & Canada size = round(([Inside diameter]÷0.001 – 11.63)÷0.8128×4)÷4
- UK & Australia size = clamp(round(([Inside circumference]÷0.001 – 37.8)÷0.625), 0, 58)
- ISO / France size = round([Inside circumference]÷0.001)
- Germany size (diameter mm) = round([Inside diameter]÷0.001×2)÷2
- Switzerland size = round([Inside circumference]÷0.001 – 40)
- Japan size = max(round(([Inside diameter]÷0.001 – 13)÷0.334) + 1, 1)
Formula
Diameter = circumference ÷ π
US size ≈ (diameter in mm − 11.63) ÷ 0.8128
UK letter: A = 37.8 mm circumference, each half size + 0.625 mm
ISO/French = circumference (mm); German = diameter (mm); Swiss = circumference − 40
Worked example
Inputs: I measured: Inside circumference / finger; Measurement: 54.4 mm.
Results: Inside diameter = 17.32 mm; Inside circumference = 54.4 mm; US & Canada size = 7; UK & Australia size = N½; ISO / France size = 54; Germany size (diameter mm) = 17.5; Switzerland size = 14; Japan size = 14.
Frequently asked questions
How can I measure my ring size at home?
Wrap a strip of paper around the finger, mark where it overlaps, measure the length in mm and enter it as circumference.
What ring size is 54 mm?
About US 6¾–7, UK N, ISO 54, German 17.25.
Should I size up for a wide band?
Yes, bands wider than about 6 mm fit tighter, so go up a quarter to half size.