How this calculator works
Ring sizes are named differently around the world, but they all describe the same physical thing: the inside diameter (or circumference) of the band. This converter takes a size in any one system — US/Canada numeric, UK/Australia letters, EU/ISO, or Japan's numbered circle sizes — and shows the equivalent in every other system, plus the underlying diameter and circumference in both millimeters and inches.
It also works as a direct lookup: if you've measured a ring's inside diameter or circumference (with a string, a strip of paper, or a printed ring sizer), enter that measurement directly to find the matching size in every system, without needing to know which system you started from.
How the systems relate
Circumference = diameter × π — a fixed geometric relationship, true regardless of sizing system.EU/ISO size (as commonly shown on jewelry size charts) = inside circumference in millimeters, rounded to the nearest whole number.US/Canada, UK/Australia, and Japan sizes are not simple formulas — they're independently defined scales, each mapped here to a published diameter/circumference table.US half-sizes, UK/Australia letters (including half-letters like N½), and Japan circle sizes in this calculator are taken from the ring size conversion table published on Wikipedia. Values between listed sizes are found by straight-line interpolation between the two nearest table entries.
Worked example: US size 7
- Look up US size 7 in the table: inside diameter = 17.32mm.
- Circumference = diameter × π = 17.32 × 3.14159 ≈ 54.41mm, which rounds to EU/ISO size 54.
- The same diameter corresponds to UK/Australia size N½ and Japan size 14 in the published table.
- In inches, the diameter is 17.32 ÷ 25.4 ≈ 0.682 inches — useful if you're comparing against a printed ring sizer marked in inches.
Frequently asked questions
Why do ring size charts from different jewelers disagree slightly?
There's no single universal ring-sizing standard used everywhere, and manufacturers round diameters and circumferences to their own conventions. The differences are usually small (well under half a US size), but they can matter for a snug fit — when in doubt, use the actual measured diameter rather than a converted size.
What's the most reliable way to find my ring size?
Measure the inside diameter of a ring that already fits well (in millimeters, if possible), or wrap a strip of paper or string around your finger and measure the circumference. Diameter and circumference are direct physical measurements and convert consistently, unlike size names that vary by region and brand.
Does finger size change during the day?
Yes — fingers are commonly slightly larger when warm (like after exercise or in hot weather) and smaller when cold. Jewelers typically recommend measuring at room temperature, and if your size falls between two options, sizing up is usually the safer choice.
What does the EU/ISO size number actually mean?
On most European and international size charts, the EU size is simply the inside circumference of the ring in millimeters. A ring labeled 'EU 54' has an inside circumference of about 54mm — no separate lookup table is needed, since it's a direct measurement.
Can I convert my ring size to a different finger's size?
No. Ring size describes the circumference or diameter of one specific finger at the time it's measured, not a proportional scale that carries over to a different finger. Because finger sizes commonly differ from each other on the same hand, each finger needs its own separate measurement.