Ring Size Converter and Calculator
Enter one thing you know: the inside diameter of a ring that fits, the circumference of your finger in millimetres, or a size in one system. The converter gives the size in US and Canada, UK and Australia, EU (ISO) and Japan, with the diameter and circumference.
Free, no sign-up. What you enter stays in your browser: it is not saved or sent to HerFound. Reviewed .
What do you know?
Measure straight across the inside of the ring, through the middle, not including the metal.
Enter a measurement or a size you know to see it in every ring size system.
How it works
Every system is a different way of labelling the same thing: the inside circumference of the ring. The converter turns what you enter into a circumference in millimetres, then reads off each system. Circumference = diameter × 3.1416.
EU sizes follow the international standard ISO 8653, where the size is simply the inside circumference in millimetres. A ring with a 54 mm inside circumference is an EU 54. US and Canadian sizes use the formula diameter in mm = 11.63 + 0.8128 × size, so each whole size adds 0.8128 mm of diameter, which is about 2.55 mm of circumference.
UK, Irish and Australian sizes are letters. On the metric British scale each letter is 1.25 mm of circumference, with size C at 40 mm, so size A is 37.5 mm and circumference = 37.5 + 1.25 × the number of letters after A. Japanese sizes are numbers: size 1 has an inside diameter of 13 mm and each size adds one third of a millimetre.
Results are rounded the way rings are sold: US to the nearest quarter size, UK to the nearest half letter, EU and Japan to the nearest whole size. The exact, unrounded value is shown next to each one so you can see which way it was rounded.
What it cannot tell you
- Conversion charts from different jewellers can disagree by a quarter or half size, because older UK and US scales were defined in fractions of an inch and makers round differently. If your size falls between two, ask the jeweller which chart they use.
- A measurement with string or paper is rough. A 1 mm error in circumference is almost half a US size. A jeweller's ring sizer, or the inside diameter of a ring that already fits, is more reliable.
- The converter gives the size of the ring, not the fit on your hand. Jewellers commonly advise going up slightly for a wide band and checking that the ring passes over your knuckle. This is common trade advice, not a measured rule.
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Common questions
To measure your ring size at home, wrap a thin strip of paper around the base of your finger, mark where it overlaps and measure the length to the mark in millimetres. That length is the circumference, which is also your EU size. You can also measure the inside diameter of a ring that fits you, straight across the middle.
A US ring size 7 is about a UK size N½ and an EU size 54. It has an inside diameter of about 17.3 mm and an inside circumference of about 54.4 mm. In Japanese sizes it is about a 14.
EU ring sizes are the inside circumference of the ring in millimetres, as set out in the standard ISO 8653. An EU size 52 ring measures 52 mm around the inside, which is a diameter of about 16.6 mm. To convert a diameter to an EU size, multiply it by 3.1416.
Jewellers commonly advise measuring your ring size at the end of the day and when your hands are warm, because fingers are slightly smaller when they are cold. Measure more than once, and check that the size also passes over your knuckle. This is general trade advice and people differ.
Wide rings usually feel tighter than narrow ones of the same size, so jewellers commonly suggest going up by about a quarter to half a size for a wide band. The exact amount depends on the ring and the maker, so ask the jeweller before ordering.