This ring size converter translates between US/Canadian, UK/Australian, European and Japanese ring sizes — plus the two measurements underneath them all: inner diameter and inner circumference in millimeters.
The reliable path to a size: measure a well-fitting ring’s inner diameter with calipers, or wrap a paper strip around the finger and mark the circumference — then read the size off this table. Fingers swell with heat and time of day; measure in the evening.
One Finger, Four Numbering Systems
Underneath every system is geometry: diameter (mm) = 0.812 × US size + 11.62, and circumference = π × diameter. Europe simply names the circumference (a "54" is 54 mm around); Japan counts from its own zero (roughly circumference − 38); the UK walks the alphabet, a letter per ~1.27 mm of circumference, halves included. Crosswalks between systems are approximate at the edges — jewelers' mandrels settle arguments. Part of our Conversion Calculators collection.
Reference Table (Most Common Sizes)
| US | Diameter (mm) | Circumference (mm) = EU | UK | Japan |
|---|---|---|---|---|
| 5 | 15.70 | 49.3 | J½ | 9 |
| 6 | 16.51 | 51.9 | L½ | 12 |
| 7 | 17.32 | 54.4 | N½ | 14 |
| 8 | 18.14 | 57.0 | P½ | 16 |
| 9 | 18.95 | 59.5 | R½ | 18 |
| 10 | 19.76 | 62.1 | T½ | 20 |
Measuring Without a Jeweler
- String/paper method: wrap snugly at the finger's base, mark, measure flat in mm — that is the circumference (= EU size, and this converter's input path via diameter = mm ÷ π).
- Existing ring: measure its inside diameter across the center with a ruler or calipers; convert diameter → US with (mm − 11.62) ÷ 0.812.
- Measure at day's end (fingers are largest), not after a cold walk or a workout; wide bands fit tighter — size up ~¼–½ for bands over 6 mm wide.
- Between sizes for a surprise purchase? The majority of resizing is painless within ±1 size; err larger for wide bands, and when in doubt a sizer kit costs less than the resizing.
Related Tools
- Shoe Size Converter — the other fit converter
- Conversion Calculators — the full collection
Frequently Asked Questions
What is a US size 7 ring in mm?
17.32 mm inner diameter, 54.4 mm inner circumference — EU size 54, UK N½, Japan 14. The worked defaults above show exactly this size.
How do I convert EU ring size to US?
EU size is the circumference in mm: diameter = mm ÷ π, then US = (diameter − 11.62) ÷ 0.812. EU 54 → diameter 17.19–17.32 mm → about US 7. Or measure your US size and read the EU value here.
How do I measure my ring size at home?
Wrap paper or string around the finger's base, mark the overlap, and measure the length in millimeters (the circumference). Compare with the table — and measure in the evening, when fingers are at their largest.
Do ring sizes differ for men and women?
The scales are unisex; average sizes differ (US women commonly 5–7, men 8–11) but the conversion math is identical for everyone.
What if I am between sizes?
For narrow bands, the smaller snug size is usually right (rings should resist slightly at the knuckle). For wide bands (6 mm+), go up a quarter to half size. Comfort-fit (domed interior) bands also wear larger than their stamped size.
Is this ring size converter free?
Yes — free, instant, no sign-up.