How to use it
- Measure the inside diameter of a ring that fits, or wrap a strip of paper around your finger to get the circumference.
- Enter the value in millimeters, or choose a US size from the list.
- Read the closest US, UK and EU sizes, and check the full table below.
How it works
Every sizing system is a different label for the same thing: the inside of the ring. US and Canadian sizes rise in steps of about 0.81 mm of diameter, starting near 11.6 mm. EU and ISO sizes are simply the inside circumference in millimeters. UK and Australian sizes use letters, with roughly one letter per half US size. The tool works out the diameter, then finds the nearest row in the chart.
diameter ≈ 11.63 + 0.8128 × US size · circumference = π × diameter
Worked example
A ring that fits measures 17.3 mm across the inside. 11.63 + 0.8128 × 7 = 17.32 mm, so that is US 7. The circumference is π × 17.32 = 54.4 mm, so EU 54, and the nearest UK size is N½.
Assumptions and limits
- Brands and countries round differently, so two charts can disagree by half a size. If you are between sizes, go up.
- Bands wider than about 6 mm fit tighter, so many jewelers suggest half a size larger.
- Fingers swell in the heat and shrink in the cold. Measure in the evening at room temperature.
Frequently asked questions
What is the EU / ISO ring size?
Under ISO 8653 the size number equals the inner circumference in millimeters, so size 54 has a 54 mm circumference.
How exact is this chart?
US sizes follow the standard diameter formula. UK letters are matched to the nearest common jeweler chart value, and brands can differ by half a size.
Can I use the on-screen ruler?
Yes. Calibrate the ruler, place a ring on the screen and read its inner diameter.