Ring Size from Circumference Calculator
Buying a ring online means knowing your size without a jeweler's set of test bands, and the most reliable home method is to measure the inside circumference of a ring that already fits well. This calculator turns that millimeter measurement into a US ring size and also reports the inside diameter. You wrap a strip of paper or a piece of string around the base of your finger, or around the inside of an existing ring, mark where it meets, and measure the length in millimeters. The tool then applies the standard US sizing scale, where each whole size adds about 2.55 mm of circumference starting from a base near 36.5 mm, so the size is the circumference minus 36.5, divided by 2.55. It also divides the circumference by pi to give the inside diameter for cross-checking against a diameter chart. Because fingers swell in heat and shrink in cold, measure when your hands are at a normal temperature, ideally in the evening, and aim for a fit that slides on with light resistance and comes off over the knuckle with a gentle tug. Every figure here is computed deterministically from your measurement, and the worked example below reconciles exactly to the calculator.
US ring size is the inside circumference minus 36.5, divided by 2.55: size = (C - 36.5) / 2.55. A 57.0 mm circumference is about US size 8.04, with an inside diameter of about 18.14 mm.
Ring size formula
US size = (C - 36.5) / 2.55
diameter = C / pi
C = inside circumference in millimeters
each whole size adds about 2.55 mm
The US scale steps by a fixed circumference per size from a base value, so subtracting the base and dividing by the step gives the size. Dividing the circumference by pi gives the inside diameter.
Worked example
An existing ring has an inside circumference of 57.0 mm.
- Subtract the base: 57.0 - 36.5 = 20.5
- Divide by the step: 20.5 / 2.55 = 8.04
- Inside diameter: 57.0 / pi = 18.14 mm
The ring is about US size 8. These are the calculator's default inputs, so the result above matches the widget exactly.
US ring size reference
| US size | Circumference (mm) | Diameter (mm) |
|---|---|---|
| 5 | 49.3 | 15.70 |
| 6 | 51.9 | 16.51 |
| 7 | 54.4 | 17.32 |
| 8 | 57.0 | 18.14 |
| 9 | 59.5 | 18.95 |
Measurement and units of measure guidance: US National Institute of Standards and Technology (NIST).
Ring size calculator: frequently asked questions
How do I find my ring size from circumference?
Measure the inside circumference of a well-fitting ring in millimeters, then apply the US scale: ring size equals circumference minus 36.5, all divided by 2.55. A 57.0 mm circumference gives about US size 8. You can also wrap a strip of paper around your finger and measure its length.
Should I measure circumference or diameter?
Either works, since they are linked by pi. Circumference is the distance all the way around the inside of the ring, and diameter is the straight line across. Diameter equals circumference divided by pi. Measuring circumference with a paper strip or string is usually easier and more accurate at home.
What is the US ring size scale based on?
US whole sizes step by about 2.55 mm of circumference each, starting from a base near 36.5 mm. That is why the formula subtracts 36.5 and divides by 2.55. Half sizes fall halfway between, so a result of 8.04 is essentially a size 8.
Does finger size change during the day?
Yes. Fingers swell in heat and after activity and shrink when cold, so size can vary by up to half a size through the day. Measure when your hands are at a normal temperature, ideally in the evening, and avoid sizing when very hot, very cold or right after exercise.
How tight should a ring fit?
It should slide on with light resistance and come off over the knuckle with a gentle tug. Too loose and it spins or slips off, too tight and it traps the knuckle. If you are between sizes, size up slightly for comfort, especially for a ring worn every day.
Official sources
- Measurement and units of measure guidance: US National Institute of Standards and Technology (NIST). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.