Ring Size Calculator
Finger measurement to US, UK, EU and Japanese sizes.
Find your ring size from a finger circumference or diameter, converted to US, UK, European and Japanese scales, with the ISO circumference in millimetres.
Wrap paper round the base of the finger and measure the overlap, or measure an existing ring across the inside.
US ring size
7.0
UK N 1/2 · 54.5 mm around
Measure late in the day at room temperature — fingers are smaller when cold and swell over the day by up to half a size. For a band wider than about 6 mm, go up a quarter to a half size.
How the Ring Size Calculator works
Ring size is one of the few clothing measurements with a genuine international standard behind it. ISO 8653 defines the size as the inside circumference in millimetres, and every national scale is a relabelling of that one number: the US scale is a straight line through it, the European scale is the same figure with forty subtracted, and the UK uses letters. Measure the circumference and every other answer follows.
Also known as: finger size calculator · ISO 8653 ring size · ring sizer conversion
How the number is derived
ISO 8653 defines a ring size as the inside circumference in millimetres. Everything else is a relabelling of that single figure. The US scale is a straight line through it: circumference in millimetres equals 36.537 plus 2.5535 times the size. A US 6 is therefore 51.9 mm around, a US 10 is 62.1 mm, and every half size is exactly halfway between.
The inside diameter, which is what a ring sizer set measures, is the circumference divided by pi. A US 6 is 16.5 mm across. Working from diameter is slightly less accurate at home because a small error in placing the calipers is multiplied by pi, which is why a paper strip around the finger is the better method if you only have one shot at it.
The European scale drops the leading forty from the ISO figure, so a 54.5 mm ring is a European 14.5. Japan runs its own numeric series from a 13 mm diameter at size 1, advancing a third of a millimetre per size.
A worked example
A paper strip wrapped around the base of the ring finger overlaps at 54.5 mm. Subtracting 36.537 gives 17.963, and dividing by 2.5535 gives 7.03 — a US 7. The inside diameter is 54.5 divided by pi, or 17.35 mm.
The same ring is a UK N½, a European 14.5 and a Japanese 13. Those four numbers describe one ring. If a jeweller in another country asks for your size, the millimetre figure is the one to give: a US 7 read as a UK 7 is two and a half sizes out, which is the difference between a ring that fits and one that falls off.
Where the answer misleads
Fingers are not a fixed size. They are measurably smaller when cold and swell over the course of a day, and the difference is enough to move half a size in either direction. Measuring a cold hand first thing in the morning is the single most common reason a ring arrives loose. Measure late in the day at normal room temperature, and if you can, measure twice on different days.
Band width changes the fit independently of the size. A wide band covers more of the finger and grips more of it, so it sits tighter than a narrow band of the same nominal size. For anything above about 6 mm, go up a quarter to a half size — the calculator cannot know the band you have chosen.
Knuckles complicate things further. A ring has to pass the knuckle and then sit on a narrower base, so someone with prominent knuckles may need a size that is loose at rest. That is a fitting judgement rather than a measurement, and it is the case where a jeweller's sizer beats any calculation.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper around the base of the finger, mark where it overlaps, and measure the length in millimetres. That length is the inside circumference, which is the ISO size directly. Measuring an existing ring's inside diameter across the middle works too.
What is the formula for US ring size?
Inside circumference in millimetres equals 36.537 plus 2.5535 times the US size. A US 6 is 51.9 mm around and 16.5 mm across; a US 10 is 62.1 mm. The relationship is linear, so half sizes are exactly halfway.
When in the day should I measure?
Late afternoon or evening, at normal room temperature. Fingers are measurably smaller when cold and swell over the day, and by enough to move a half size. Measuring a cold hand first thing is the most common reason a ring arrives too loose.
Should I size up for a wide band?
Usually by a quarter to a half size. A wide band covers more of the finger and sits more tightly than a narrow one of the same nominal size, which is why the same person can wear a 6 in a 2 mm band and need a 6.5 in an 8 mm one.
How do UK letter sizes relate to US numbers?
The UK scale advances in half-letter steps at four steps per US size, so US 6 is L½, US 7 is N½ and US 9 is R½. It is a relabelling of the same circumference rather than a different measurement.