Ring Size Calculator
Find your ring size from the inside circumference or diameter of a ring that fits, or from a string wrapped around your finger, and convert between US, UK, EU/ISO (mm) and Japanese sizes. Standard measurements, no brand tables.
Last updated: October 2026
Ring size
7 US
Formula below · 3 sources (NIST, gemsociety.org, tiffany.com) · Updated Oct 2026
Compare with similar
About this calculator
Every ring size system is a way of writing the inside circumference (or diameter) of the ring, so the calculator converts your input to the inside circumference in millimetres and reads every size off that one number.
• US and Canadian sizes: inside diameter = 0.458 in + 0.032 in per size (11.63 mm + 0.8128 mm per size). A US 7 is 17.3 mm across and 54.4 mm around; each whole size adds 2.55 mm of circumference, a half size 1.28 mm. • UK, Irish, Australian and South African sizes use letters: A is 37.8 mm around and each letter adds 1.27 mm (half letters, written N½, add 0.635 mm). US 7 is a UK N. • ISO 8653 (the European, French and "Vienna" scale) is simply the inside circumference in millimetres: a size 54 ring is 54 mm around. German jewellers may quote the same ring as 17.3, its diameter. • Japanese sizes start at 13 mm diameter for size 1 and add ⅓ mm per size, so a US 7 is a Japanese 14.
Measuring. The most reliable input is the inside diameter of a ring that already fits the intended finger, measured with a ruler or calipers across the widest inside point, chosen as "Inside diameter in mm". The string method works too: wrap a thin strip of paper or string snugly where the ring will sit, mark where it overlaps, measure the length in millimetres and choose "Inside circumference in mm". Measure at the end of the day when fingers are largest, not when cold, and do it two or three times. Knuckles matter: if the ring must pass a large knuckle, size for the knuckle and accept a slightly loose fit at the base. Wide bands (6 mm and up) fit tighter than thin ones, so go up a half size for them.
Results are rounded to the nearest half size (US, UK, EU) or whole size (Japan); the unrounded US size appears in the tiles so you can see whether you sit between sizes. Quarter sizes exist at some jewellers and are the right choice when the unrounded size ends in .25 or .75.
Ring size conversion chart (US 3 to 13)
Scroll sideways to see all columns →
| US / Canada | Inside diameter (mm) | Inside circumference (mm) | UK / AU | EU / ISO | Japan |
|---|---|---|---|---|---|
| 3 | 14.07 | 44.2 | F | 44 | 4 |
| 3.5 | 14.48 | 45.5 | G | 45.5 | 5 |
| 4 | 14.88 | 46.8 | H | 47 | 7 |
| 4.5 | 15.29 | 48.0 | I | 48 | 8 |
| 5 | 15.70 | 49.3 | J | 49.5 | 9 |
| 5.5 | 16.10 | 50.6 | K | 50.5 | 10 |
| 6 | 16.51 | 51.9 | L | 52 | 12 |
| 6.5 | 16.92 | 53.1 | M | 53 | 13 |
| 7 | 17.32 | 54.4 | N | 54.5 | 14 |
| 7.5 | 17.73 | 55.7 | O | 55.5 | 15 |
| 8 | 18.14 | 57.0 | P | 57 | 16 |
| 8.5 | 18.54 | 58.3 | Q | 58.5 | 18 |
| 9 | 18.95 | 59.5 | R | 59.5 | 19 |
| 9.5 | 19.35 | 60.8 | S | 61 | 20 |
| 10 | 19.76 | 62.1 | T | 62 | 21 |
| 10.5 | 20.17 | 63.4 | U | 63.5 | 23 |
| 11 | 20.57 | 64.6 | V | 64.5 | 24 |
| 11.5 | 20.98 | 65.9 | W | 66 | 25 |
| 12 | 21.39 | 67.2 | X | 67 | 26 |
| 12.5 | 21.79 | 68.5 | Y | 68.5 | 27 |
| 13 | 22.20 | 69.7 | Z | 69.5 | 29 |
How to use
Measure, pick what the number is, and pick the size system you want. The tiles show the diameter, circumference and the other systems at once. Example 1 (defaults): a 54.4 mm string measurement. Diameter = 54.4 ÷ π = 17.32 mm; US size = (17.32 − 11.63) ÷ 0.8128 = 7.0 → US 7 (UK N, EU 54.5, Japan 14). Example 2: a ring that fits is 16.5 mm across inside. Circumference = 16.5 × π = 51.8 mm; US size = (16.5 − 11.63) ÷ 0.8128 = 5.99 → US 6, UK L, EU 52. Example 3: a UK partner wears a US 10. Circumference = π × (11.63 + 8.128) = 62.1 mm; UK index = (62.1 − 37.83) ÷ 1.27 = 19.1 → letter T. Example 4: a Japanese 9. Diameter = 12.667 + 3 = 15.67 mm; circumference 49.2 mm; US 4.97 → US 5, UK J.
Frequently asked questions
How do I measure my ring size with a string?
Wrap a strip of paper or non-stretch string around the base of the finger, snug but able to slide over the knuckle, mark the overlap, and measure the length in millimetres with a ruler. Enter that as the inside circumference. Repeat two or three times and use the average; a 1 mm error is about half a US size.
What is a size 7 ring in mm?
A US 7 is 17.3 mm inside diameter and 54.4 mm inside circumference. In UK letters it is an N, in ISO/EU sizing a 54 or 54.5, and in Japan a 14.
Are US and UK ring sizes the same?
No. The US uses numbers (a half size is 0.41 mm of diameter) and the UK uses letters (a half letter is 0.2 mm). Each US size is two UK letters: US 5 = J, US 6 = L, US 7 = N, US 8 = P, US 9 = R, US 10 = T, US 11 = V, US 12 = X, with half sizes falling on the letters in between.
Should I size up for a wide band?
Yes: a band 6 mm or wider covers more of the finger and feels tighter, so most jewellers suggest a half size larger than for a thin band. Measure with a ring of similar width when you can.
Why does my size change during the day?
Fingers swell with heat, salt, exercise and in the evening, and shrink in the cold; the difference can be a full size. Measure at a normal room temperature late in the day, and aim for a fit that slides over the knuckle with slight resistance and does not spin freely at the base.
What are the EU or ISO sizes on European rings?
ISO 8653 defines the ring size as the inside circumference in millimetres, so EU 52 is 52 mm around (a US 6) and EU 60 is 60 mm around (a US 9). German sizes are sometimes the inside diameter instead (17 = 17 mm across ≈ US 6.75); if the number is below 25 it is a diameter.
Related calculators
Sources & references
Think a result is wrong? Report a problem with this calculator.