Ring Size Calculator
Find your ring size from a measurement — enter the inside diameter or circumference in millimeters to get your US, UK and EU ring size, with a guide to measure at home.
How to measure your ring size at home
You need one measurement in millimeters: either the inside diameter of a ring that already fits, or the circumference of your finger. To measure your finger, wrap a thin strip of paper or a piece of string snugly around the base, mark where it overlaps, and measure that length with a ruler. To measure a ring, lay it flat and measure straight across the inside. Enter the number above, pick which one you measured, and the calculator gives your US, UK and EU size.
US size = (circumference in mm − 36.537) ÷ 2.5535, circumference = π × diameter
For example, a ring with a 17 mm inside diameter has about a 53.4 mm circumference, which is a US size 6.5 (UK M½, EU 53). Measure at the end of the day when your fingers are largest, and take the average of two or three tries for the best result.
US, UK and EU ring sizes
The three systems all describe the same physical size, just labeled differently. US sizes are numbers with half sizes, UK sizes are letters (A–Z), and EU sizes are simply the inside circumference in millimeters. That is why a single measurement converts cleanly to all three.
| Diameter (mm) | US | UK | EU |
|---|---|---|---|
| 15.7 | 5 | J½ | 49 |
| 16.5 | 6 | L½ | 52 |
| 17.3 | 7 | N½ | 54 |
| 18.2 | 8 | P½ | 57 |
| 19.8 | 10 | T½ | 62 |
This is an estimate — paper and string can stretch, so for an expensive ring confirm with a jeweler’s sizing tool before buying. Converting other units? See the length converter.
Frequently asked questions
How do I measure my ring size at home?
Wrap a strip of paper or string snugly around the base of your finger, mark where it overlaps, and measure that length in millimeters — that is the circumference. Or measure the inside diameter of a ring that already fits. Enter either measurement above to get your size.
What is my ring size if the diameter is 17 mm?
An inside diameter of about 17 mm is roughly a US size 6.5 (circumference ≈ 53.4 mm, UK size N, EU 53). The calculator converts your exact measurement — enter the diameter or circumference in millimeters for the US, UK and EU size.
What is the difference between US and UK ring sizes?
US ring sizes use numbers (with half sizes), while UK sizes use letters (A–Z). Both are based on the inside circumference, just labeled differently, and EU sizes are simply the circumference in millimeters. The calculator shows all three from one measurement.
How accurate is measuring ring size with string?
String or paper gives a good estimate but can stretch or sit loosely, so measure a few times and take the average, and measure at the end of the day when fingers are largest. For an expensive ring, confirm with a jeweler’s sizing tool before buying.