Type a measurement, get your ring size. Enter the inside diameter of a ring that fits, or the circumference of your finger from a string or paper strip, in millimetres or inches. The calculator rounds to the nearest standard size and shows it in US, UK, EU/ISO and Indian sizes. Example: a finger that measures 54.3 mm around is a US 7 (UK N½, EU 54).
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How to use the ring size calculator
- Pick what you measured. “Inner circumference” is the length of a string or paper strip wrapped around your finger. “Inner diameter” is the distance straight across the inside of a ring you already own.
- Type the number in mm, or choose the inch option if your ruler is in inches. Decimals are fine: 17.6 mm or 0.69 in.
- Read the result. The calculator picks the closest standard size. If your number is right between two sizes, the larger one is usually safer, because a ring must still pass over the knuckle.
No measurement yet? Switch to “Match a ring I own”, calibrate with a bank card and place a ring on your screen, or follow the steps in how to measure ring size at home.
The formula behind the calculator
US and Canadian ring sizes grow by a fixed step of about 0.81 mm of inner diameter per whole size. Jewellers’ charts follow this line closely:
- Inner diameter (mm) = 11.63 + 0.8128 × US size
- US size = (inner diameter − 11.63) ÷ 0.8128
- Diameter = circumference ÷ π (π = 3.1416), and circumference = diameter × π
- EU / ISO size = the inner circumference in mm, rounded
The calculator does these steps for you and then matches the result to the standard chart (rather than printing a long decimal), because rings are only made in half and quarter sizes.
Worked examples
| You measured | Diameter | Calculation | Ring size |
|---|---|---|---|
| Finger circumference 57 mm | 57 ÷ 3.1416 = 18.14 mm | (18.14 − 11.63) ÷ 0.8128 = 8.0 | US 8 / UK P½ / EU 57 |
| String around finger 2.2 in | 55.9 mm ÷ 3.1416 = 17.79 mm | (17.79 − 11.63) ÷ 0.8128 = 7.58 | US 7.5 / UK O½ / EU 56 |
| Ring inner diameter 16.5 mm | 16.5 mm | (16.5 − 11.63) ÷ 0.8128 = 5.99 | US 6 / UK L½ / EU 52 |
| Ring inner diameter 0.75 in | 0.75 × 25.4 = 19.05 mm | (19.05 − 11.63) ÷ 0.8128 = 9.13 | US 9 / UK R½ / EU 60 |
| Finger circumference 62 mm | 62 ÷ 3.1416 = 19.74 mm | (19.74 − 11.63) ÷ 0.8128 = 9.98 | US 10 / UK T½ / EU 62 |
For the full list of measurements and sizes, see ring size by circumference and ring size by diameter.
Which measurement is more accurate?
Measuring the inside of a ring that already fits is the most reliable home method, because a metal ring does not stretch and already reflects how tight you like it. A string or paper strip around the finger can be off by half a size: string stretches, paper slips, and it is easy to pull too tight. If you only have a finger measurement, measure two or three times and use the average. When the calculator says you are between two sizes, choose the larger one for a wide band and the smaller one for a thin band you will wear daily.
Calculator, on-screen sizer or chart?
- This calculator is quickest when you already have a number from a ruler, caliper or tape.
- The on-screen ring sizer is quickest when you have a ring but no ruler: lay it on your phone and match the circle.
- The ring size converter is for when you already know your size in one system (say UK N) and need it in another (US or EU).
- The printable ring sizer gives you paper circles and a finger strip if you prefer to measure away from a screen.
Things the calculator cannot know
- Band width. Bands wider than about 6 mm fit tighter. Add ¼ to ½ size.
- Your knuckle. If the knuckle is much larger than the base of the finger, measure both and choose a size between them.
- Temperature and time of day. Fingers are smaller in the morning and in the cold. Measure in the evening at room temperature.
- Which hand. The dominant hand is often ¼ to ½ size larger, so measure the finger the ring is for.
Not sure of your ring size? Measure it on your phone screen in 30 seconds.
Open the free ring sizer →Frequently asked questions
How do I calculate my ring size from mm?
If you measured a ring’s inner diameter, find the nearest value in the chart or use US size = (diameter − 11.63) ÷ 0.8128. If you measured around your finger, divide by 3.1416 first to get the diameter. A 17.3 mm diameter or 54.3 mm circumference is US 7.
What ring size is 55 mm circumference?
55 mm around is a diameter of 17.5 mm, between US 7 (17.3 mm) and US 7.5 (17.7 mm). Most people choose US 7.5, UK O½, EU 55–56.
What ring size is 2 inches around?
2 inches is 50.8 mm of circumference and 16.2 mm of diameter, which is closest to US 5.5 (UK K½, EU 51).
Is it better to round a ring size up or down?
Round up when you are exactly between two sizes, when the band is wide, or when your knuckle is large. Round down only for a thin band on a slim knuckle that spins easily.
How accurate is a ring size calculator?
The maths is exact; the result is only as good as the measurement. A ring measured with a caliper gives about ±¼ size. A string around the finger is usually within ±½ size.