How is ring size related to diameter?

Ring size systems are based on the inner diameter of the band. US sizes increase by about 0.8 mm of diameter per half size. The EU (ISO 8653) size is simply the inner circumference in millimetres rounded, so an EU 54 ring has a circumference of roughly 54 mm.

Should I measure diameter or circumference?

Circumference is easier to measure accurately on a finger using paper or string. If you have an existing ring, measure its inside diameter with a ruler. Circumference equals diameter times pi (about 3.1416), and this tool converts between the two automatically.

Do US and UK ring sizes match?

No. They use completely different scales. A US size 6 is roughly a UK L 1/2, and a US 7 is about a UK N 1/2. Always convert rather than assuming the numbers or letters line up between countries.

What if my measurement falls between two sizes?

Round up to the next size, especially for wide bands which fit more snugly, and for fingers that swell in heat. A slightly loose ring is far easier to wear than one that will not pass the knuckle.

Are these sizes the same for men and women?

Yes. Ring sizing is based purely on finger diameter, not gender. Men typically wear US sizes 8 to 13 and women US 4 to 9, but the conversion table itself is identical for everyone.

Ring Size Conversion Chart

Email me this result

Get this tool's output sent to your inbox, plus one useful tool a week. No spam, unsubscribe any time.

Ring sizes look simple until you cross a border: the United States uses numbers, the UK and Australia use letters, and most of Europe uses the inner circumference in millimetres. This chart maps all three to the underlying physical size of the band so you can convert between any of them, or find your size from a measurement.

How it works

Every ring size ultimately describes one number: the inner diameter of the band. The other systems are derived from it.

circumference = diameter × π          (π ≈ 3.1416)
EU size       ≈ circumference (mm), rounded   (ISO 8653)
US size       ≈ (diameter − 11.63) / 0.8128 + 0.5  (approx.)

The lookup uses a fixed conversion table built from the standard ISO and US sizing scales. When you enter a measured diameter or circumference, the tool finds the row whose diameter is closest to your value and reports every equivalent. Because string-and-paper measurements are imperfect, treat the nearest match as a starting point.

Tips and notes

Measure at the end of the day, when fingers are at their largest.
Cold fingers shrink — never size a ring on a chilly morning.
For bands wider than about 6 mm, go up a half size, as wide bands feel tighter.
A common rule: the ring should slide on with mild resistance and require a gentle tug to come off over the knuckle.

For example, an inner diameter of 17.3 mm gives a circumference of about 54.3 mm, matching EU 54, roughly US 7, and around UK N 1/2.