What this calculator does
Find your ring size in the US, UK, European and Japanese systems from one measurement or one known size. A ring size is really just a code for how big the hole in a ring is. The trouble is that each region uses its own code. The United States and Canada use numbers, from about 3 to 13 for most adults. The UK and Australia use letters. Most of mainland Europe uses the distance around the inside, in millimetres, and Japan has its own numbered scale.
Start from whatever you have. That could be the distance around your finger, the width across the inside of a ring that already fits, or a size from one of the systems. The converter gives the matching size in each of the others. It also shows the inside width (diameter) and the distance around (circumference) in millimetres.
Where it's used
Why this math matters outside the classroom:
- Buying online. Shoppers measure an old ring's inside diameter, such as 17.3 mm, and find it is close to a US size 7.
- String method. A string wrapped around a finger measuring 54 mm gives a diameter of 54 ÷ π ≈ 17.2 mm.
- Gifts abroad. Buyers convert a US size to a UK letter or EU number before ordering from a jeweler in another country.
- Resizing a band. Jewelers know each US size adds about 0.8128 × π ≈ 2.55 mm to a ring's inside circumference.
- Circumference practice. Students use C = πd to turn an 18 mm inside diameter into a circumference of about 56.5 mm.
Grade level and standards: Grade 7; Common Core State Standards for Mathematics 7.G.B.4. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose what you know: a circumference, an inner diameter, or a US, EU or Japanese size.
- For a measurement, enter the number and pick millimetres, centimetres or inches.
- Read the US size first, then the UK, EU and Japanese sizes that match it.
- If you're between sizes, the working shows the unrounded value, so you can see which way you lean.
What the inputs mean
- Measurement
- For a finger, wrap a thin strip of paper or string snugly around the base. Mark where it overlaps and measure it flat; that's the circumference. For a ring, measure straight across the inside, from edge to edge; that's the inner diameter. Measure at the end of the day, when fingers are slightly larger, and make sure the ring can still pass over your knuckle.
- EU / ISO ring size
- The European size (from the ISO 8653 standard) is the distance around the inside in millimetres, so size 54 is 54 mm around. Some countries, such as Italy, Spain and Switzerland, often quote this number minus 40 instead.
- Japanese ring size
- Japanese sizes are whole numbers. Each one is about a third of a millimetre wider across than the one before.
Formula and method
inner circumference C = π × inner diameter D US / Canada size = (D − 11.63 mm) ÷ 0.8128 mm EU / ISO size = C in millimetres UK / Australian letter = A + (C − 37.5 mm) ÷ 1.25 mm letters Japanese size ≈ 3 × D − 38 (D in mm)
In the formulas, D is the inner diameter (the width across the inside of the ring) and C is the inner circumference (the distance around the inside). π (pi) is about 3.1416, the number that links any circle's width to its distance around. So if you know one, you know the other. In the UK formula, A is the first letter of the scale, and the result says how many letters to count on from A. Every scale measures the inside of the ring.
On the US and Canadian scale, size 0 is 0.458 inches (11.63 mm) across. Each whole size adds 0.032 inches (0.8128 mm) to that width, which is about 2.55 mm more around. Half and quarter sizes split that step evenly. Results are rounded to the nearest quarter size.
The UK and Australian letters go up by about 1.25 mm of circumference per letter, starting near 37.5 mm for A. Jewellers also sell half letters. The Japanese scale goes up by about 1/3 mm of diameter per size, with size 1 at 13 mm.
Most published conversion charts follow these rules. But charts from different jewellers disagree by a fraction of a size. Treat the results as close estimates, and confirm with a ring sizer or a ring that already fits.
Worked example
Someone wears a US size 7 and wants to order from a UK, a European and a Japanese seller.
- Inner diameter = 11.63 + 0.8128 × 7 = 17.32 mm.
- Inner circumference = π × 17.32 = 54.41 mm, so the EU / ISO size is about 54.
- UK letter: (54.41 − 37.5) ÷ 1.25 = 13.53 steps after A, which rounds to 13½, the letter N½.
- Japanese size: 3 × 17.32 − 38 = 13.96, about size 14.
Result: A US 7 is roughly UK N½, EU 54 and Japanese 14, with an inside diameter of about 17.3 mm.
Understanding the result
If your measurement falls between two sizes, choose the larger one in two cases. One is a wide band (roughly 6 mm or more), which feels tighter. The other is when your knuckle is bigger than the base of your finger.
Fingers swell in the heat and shrink in the cold. So a measurement taken on a cold morning may come out half a size small.
The EU size and the circumference are the same number, rounded to the nearest millimetre.
Assumptions and limitations
- The measurement is of the inside of the ring, or the snug distance around the finger where the ring will sit.
- Sizes follow the common US, ISO 8653, UK letter and Japanese chart rules described above.
- Charts vary slightly between jewellers and countries, so results are approximate.
- The range covers US sizes 0 to 16. The calculator turns away measurements outside that range.
- Comfort-fit bands (rounded on the inside) and very wide bands may need a different size from a standard band with the same inside width.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper or a piece of string that doesn't stretch around the base of your finger. Mark where it meets, then measure that length in millimetres. Enter it as the circumference. Or measure straight across the inside of a ring that fits, and enter that as the diameter.
What is the most common ring size?
There's no single standard, but most adult ring sizes fall between about US 5 and US 11. Fingers differ between hands, and even between fingers on the same hand. So measure the finger the ring is for.
Is a European ring size the same as millimetres?
Under ISO 8653, yes: the size is the distance around the inside in millimetres. Some shops in Italy, Spain and Switzerland subtract 40, so an ISO 54 may be shown as size 14 there.
How much bigger is one ring size?
One whole US size adds about 0.81 mm to the inside width, or about 2.55 mm to the distance around. One UK letter adds about 1.25 mm around, roughly half a US size.
How this calculator is checked
Method: Common jewellers' chart conventions: US size 0 = 11.63 mm diameter plus 0.8128 mm per size; ISO 8653 circumference sizing; UK letters from 37.5 mm in 1.25 mm steps; Japanese sizes 1/3 mm of diameter apart. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
Embed this calculator
Put this Ring Size Converter on your own website or blog for free. Paste this code where you want it to appear: