About the mm to Ring Size Converter
This tool converts a millimeter measurement of a ring — either its inner diameter or its inner circumference — into the three ring-sizing systems used around the world: US/Canada (a plain number), EU/ISO (the circumference itself, per ISO 8653), and UK/Australia/Ireland (a letter from A to Z). It works from ordinary circle geometry plus the linear formulas jewelers use to map circumference onto each scale.
The formulas
Every result on this page is built from the ring's inner circumference, so the first step is always to get that figure in millimeters:
- Circumference from diameter: Circumference = pi x Diameter, using pi ~ 3.14159. This is plain circle geometry — the inner edge of a ring is a circle, and its circumference is what has to fit around the base of the finger.
- US / Canada size: US size = (Circumference in mm − 36.5) / 2.55. This linear formula matches the widely published US sizing chart to within about 0.05 of a size across the common range (roughly size 3 to size 13).
- EU / ISO size: ISO 8653 defines a ring's size as its inner circumference in millimeters, so the EU/ISO number is simply the circumference, rounded to the nearest whole millimeter.
- UK / Australia / Ireland size: this scale runs alphabetically from A to Z, with each full letter representing roughly a 1.25 mm increase in circumference starting near 37.8 mm at "A". Half sizes fall exactly halfway between two letters.
How to take the measurement
There are two practical ways to get a millimeter figure to enter above:
- Measure an existing ring: lay a ring that already fits flat on a table and measure straight across the inside edge — rim to rim, not including the metal — with a ruler or digital calipers. That is the inner diameter.
- Measure the finger directly: wrap a thin strip of paper, string, or a cloth tape snugly (not tight) around the base of the finger where the ring will sit, mark the point where it overlaps, then lay it flat and measure that length. That is the inner circumference.
Reference points
Using the formulas above, a few common sizes work out as follows:
- US 5 ≈ 15.68 mm diameter / 49.25 mm circumference ≈ EU 49 / UK J
- US 7 ≈ 17.30 mm diameter / 54.35 mm circumference ≈ EU 54 / UK N
- US 9 ≈ 18.92 mm diameter / 59.45 mm circumference ≈ EU 59 / UK R 1/2
- US 11 ≈ 20.55 mm diameter / 64.55 mm circumference ≈ EU 65 / UK V 1/2
Ring sizing is not perfectly standardized between manufacturers, and wider bands feel tighter than narrow ones at the same measured size, so treat every converter's output — including this one — as a close estimate rather than a guaranteed fit. For a final purchase, especially for an engagement or wedding ring, confirm with a physical ring sizer or a jeweler.