Ring Size Converter
This ring size converter turns an inner circumference or diameter in millimetres into the matching size in the ISO/EU, US, UK, Swiss and Japanese systems, so you can order from any shop.
Source: Wikipedia – Ring size (ISO 8653, US-Formel d = 0,8128 · s + 11,63, britische Buchstaben, Schweizer Skala). Updated: .
How it is calculated
An inner circumference of 54 mm is ISO size 54, US 6¾, UK N and Swiss size 14. The ISO size is simply the inner circumference of the ring in millimetres.
How it is calculated
The core quantity is the inner circumference C. The diameter is d = C / π. The ISO size (ISO 8653, used in much of continental Europe) equals C in mm. The US size s follows from d = 11.63 + 0.8128 · s mm in quarter steps, the British letter size from C = 37.5 + 1.25 · i mm (A = 37.5 mm, C = 40 mm). The Swiss, Italian and Spanish size is C − 40 mm, and the Japanese size starts at 13 mm inner diameter and grows in steps of 1/3 mm.
Good to know
- The most accurate method is a ring that already fits: measure its inner diameter and enter it here.
- Fingers swell with heat and exercise and are slimmer in the morning. Several measurements on different days add confidence.
- The converter shows the nearest standard size and its exact circumference; jewellers sometimes use slightly different charts.
- Your input stays in the browser.
Frequently asked questions
How do you find your ring size?
Wrap a paper strip or string snugly around the finger, mark the overlap and measure the length in mm. Enter that circumference here to see every size system.
How do you convert mm to ring size?
In the ISO system the size is the inner circumference in mm. For the diameter divide by π (3.1416): a 54 mm circumference is a 17.2 mm diameter.
What is US size 7 in EU sizes?
US 7 is about 54.4 mm inner circumference, so ISO/EU size 54, or 55 for a roomier fit.
What is ring size 52 in cm?
Size 52 means a 52 mm inner circumference, i.e. 5.2 cm, and about 16.6 mm inner diameter.
Sources and legal basis
- Wikipedia – Ring size (ISO 8653, US-Formel d = 0,8128 · s + 11,63, britische Buchstaben, Schweizer Skala)
- ISO 8653:2016 – Jewellery and precious metals: Ring-sizes (Größe = Innenumfang in mm)
- Wikipedia – 指輪 (日本の号数: 内径13 mmが1号、1/3 mmごとに1号増)
As of: · Editorial policy
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