How ring sizes are defined
Every country measures the same thing, the inside of the ring, but numbers it differently:
| System | Rule |
|---|---|
| US / Canada | Inner diameter (mm) = 11.63 + 0.8128 × size, in quarter sizes |
| UK / Australia | Letters; C = 40 mm circumference, each letter adds 1.25 mm |
| Europe (ISO 8653) | The size is the inner circumference in mm |
| Korea / Japan | Size 1 = 13.0 mm inner diameter, each size adds 1/3 mm |
That is why a US 7 is roughly a UK N½, a European 54 and a Korean or Japanese 14. Shops do not always round the same way, so every result here also shows the millimetre value to check against the seller’s own chart.
Measuring with a paper strip
Wrap a thin strip of paper around the base of the finger, mark where it overlaps, then lay it flat and measure it with a ruler. That is your finger circumference. Say it comes to 57 mm: the diameter is 57 ÷ 3.1416 = 18.14 mm, which works out to US 8, UK P½, EU 57 and Korean size 16. If you land between two sizes, go up. String works too, but it stretches and tends to read small.
Measuring a ring on your screen
No ruler? Calibrate the screen first by holding a standard bank or ID card (85.60 × 53.98 mm) against the box and resizing it until it matches. Then place a ring that fits on the circle and grow the circle until its edge lines up with the inside edge of the ring. The calibration is saved on this device, and the tool asks you to redo it if your browser zoom changes.
Getting it right
- Fingers swell in the evening, in heat and after exercise. Measure a couple of times and use the larger figure.
- If your knuckles are larger than the base of your finger, the ring has to get over the knuckle, so check that too.
- Wide bands (over about 6 mm) feel tighter; most people size up by half to one size.
- Buying jewellery while living in Korea? Shops quote sizes as 호 (ho), which is the Korean/Japanese scale in the chart.
Need to turn inches into millimetres first? Use the Unit Converter.