If you have ever shopped for a ring from another country, you have probably hit this problem: the seller lists sizes like "N", "54", or "14", and none of it matches the number you know. Ring sizes are not one scale with different labels. Different regions measure different things, in different steps. Understanding the logic behind each system makes conversions far less confusing, and helps you spot when a chart is giving you only an approximation.
The two things every system is really measuring
A ring is a circle, so every size boils down to one of two physical measurements: the inner diameter(the width across the inside of the ring) or the inner circumference (the distance around the inside). The two are linked by simple geometry: circumference equals diameter multiplied by pi (about 3.1416). A ring with a 17.3 mm inner diameter has an inner circumference of about 54.4 mm. That is a US size 7, a UK size N, and roughly an EU size 54. The systems differ only in how they label and step through those measurements.
United States and Canada
The US scale uses numbers with half-size steps, such as 6, 6.5, 7. Each full size adds about 0.8 mm of inner diameter, or roughly 2.5 mm of circumference. A size 7 sits at about 17.3 mm diameter. Most women's rings are sold between about 5 and 8, and most men's between about 8 and 12, though plenty of people fall outside those ranges. The US scale is the one most online retailers use by default, which is why most conversion charts treat it as the reference.
United Kingdom, Australia and Ireland
These countries use letters, again with half sizes (for example N and N½). A UK "N" matches a US 7, "P" matches a US 8, and so on. Because letters carry no obvious measurement, a UK size is hard to estimate in your head. You need a chart or a converter. South Africa and New Zealand commonly use the same lettered scale.
Europe and the ISO standard
The international standard, ISO 8653, defines ring size as the inner circumference in millimeters, rounded to whole numbers. An EU size 54 therefore means a circumference of about 54 mm. This is the most intuitive system because the number is the measurement. It is also why a strip of paper or string wrapped around your finger can be converted almost directly: measure the circumference in millimeters and you are close to your EU size.
One caution: some European countries and individual jewelers use an offset version of the circumference scale (for example, circumference minus a fixed amount). If a seller's numbers look much smaller than expected, check which convention they use before ordering.
Japan
Japanese sizes are whole numbers that increase in small diameter steps of roughly one third of a millimeter. Size 1 is about 13 mm across, and a size 14 comes out near 17.3 mm, which lines up with a US 7. Because the steps are finer than US half sizes, Japanese sizes can distinguish between fits that other systems would round together.
India and China
India and China each use their own numeric scales. The numbers do not match US or UK ones, and published charts for these regions are not always identical, since local jewelers sometimes follow slightly different conventions. For these two systems especially, treat any conversion chart (including ours) as a starting point, and confirm the size against the seller's own chart or against a physical measurement in millimeters when the purchase is expensive.
Quick reference
| US | UK | EU (ISO) | Inner diameter | Inner circumference |
|---|---|---|---|---|
| 5 | J | 49 | 15.7 mm | 49.3 mm |
| 6 | L | 51 | 16.5 mm | 51.9 mm |
| 7 | N | 54 | 17.3 mm | 54.4 mm |
| 8 | P | 56 | 18.1 mm | 57.0 mm |
For every half size across all six regions, see the full ring size chart.
How to convert reliably
- Measure in millimeters first (diameter or circumference), not in a regional size number.
- Convert the millimeter value to the size system you need, rather than converting from one size label to another.
- If your measurement falls between two sizes, choose based on the ring's width and your knuckle (see our guide to wide band sizing).
- For a high-value or custom ring, confirm with the seller's own chart, since brands sometimes use their own numbering.
That last point matters most for brand-specific scales. For example, our guides toCartier and Oura sizing show how a brand's own numbers can differ from the standard system.