US
inside Ø = 11.63 + 0.8128 × size
Measured, not guessed
Ring Size Chart & Converter — US, UK, EU, Japan
Ring sizing has no single international standard. The US counts in numbers, the UK in letters, the EU in millimetres of circumference and Japan in 号 — and each was defined separately, so they do not land on each other's whole numbers. Printed charts hide that by rounding, which is how two reputable jewellers can name different sizes for the same finger. This converter works from the one thing all four agree on — the inside measurement of the finger — and shows both the exact figure and the size you would actually be sold.
The four definitions
UK — BS 6820
circumference = 37.8 mm + 1.25 mm per letter
Japan / Korea — JIS S 4700
inside Ø = 13.0 + (size − 1) ÷ 3
What this is good for
Buying from another country
A UK jeweller's letter and a Japanese 号 describe the same finger through different arithmetic. Converting through millimetres avoids inheriting someone else's rounding.
Sizing a gift without asking
Measure the inside of a ring they already wear and enter the circumference. Diameter × π gives the number this tool wants.
Checking a chart you were given
Enter your size and compare the exact column with the chart in front of you. A half-size difference usually means the chart rounded, not that one of you is wrong.
Frequently asked questions
Why do two ring size charts give me different answers?
Because the systems are defined independently and rarely coincide on whole numbers. A US 6 works out to Japanese 11.5, so one chart prints 11 and another prints 12. Both are rounding the same measurement in different directions.
Is EU ring size the same as circumference in millimetres?
Yes. Under ISO 8653 the European size is simply the inside circumference in millimetres, which is why a size 54 ring measures 54 mm around the inside.
Is the Korean 호 the same as the Japanese 号?
They use the same scale. Korean jewellers size on the JIS series, where size 10 is exactly 16.0 mm inside diameter.
String or paper for measuring?
Paper. String stretches under tension and consistently reads a size small. Measure at the base of the finger, in the evening, and take the larger size if you fall between two.
Sources and method
US sizes follow the standard inside-diameter series (11.63 mm at size 0, 0.8128 mm per size), which puts size 7 at 17.32 mm diameter and 54.4 mm circumference — matching the figure jewellers publish. UK letters follow BS 6820 (1987), which defines sizes by inside circumference starting at 37.8 mm for A and rising 1.25 mm per letter. Japanese and Korean sizes follow JIS S 4700. European sizes follow ISO 8653, where the size is the circumference in millimetres.
This is a conversion aid, not a fitting. Band width, knuckle size and temperature all change how a ring sits, so have anything expensive sized in person.