How this ring size converter works
Ring size describes how big the inside of the band is, so it is measured from the inside — either as the inside diameter (the straight-line distance across the centre) or the inside circumference (the distance all the way around). The two are linked by a constant: circumference equals diameter multiplied by about 3.14 (π).
This tool takes whichever millimetre figure you have and finds the closest standard size, then shows the matching US, UK, EU (ISO) and Japan values. The EU number is simply the inside circumference in millimetres, which is why a US 7 (about 54.4 mm around) is EU 54.4. You can also work the other way — pick a US size and the millimetre and international equivalents fill in automatically.
Reference note: values follow the standard jewellers' conversion chart (ISO 8653 for the EU circumference sizes). Anchors: US 6 ≈ 16.5 mm / 51.9 mm, US 7 ≈ 17.3 mm / 54.4 mm, US 8 ≈ 18.1 mm / 57.0 mm, US 9 ≈ 18.9 mm / 59.5 mm, US 10 ≈ 19.8 mm / 62.1 mm.
How to measure your ring size at home
- String or paper method: wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, lay it flat and measure the length in millimetres. That length is your circumference — enter it above.
- Existing ring method: take a ring that already fits the right finger and measure the inside diameter straight across the centre in millimetres, then switch the tool to "Diameter".
- Timing: measure at the end of the day at a normal temperature. Fingers swell in heat and after activity, and shrink when cold, which can shift the result by a size.
Measure two or three times and average the figures. For wide bands (about 6 mm or more), consider going up a quarter to a half size for comfort.
Ring size chart (US / UK / EU / mm)
Use this chart as a quick reference. The diameter is the inside measurement across the band and the circumference is the distance around the inside — which is also the EU (ISO) size.
Frequently asked questions
- How do I measure my ring size at home?
- Wrap a thin strip of paper or string snugly around the base of the finger, mark the overlap and measure that length in millimetres for the circumference, or measure the inside diameter across the centre of a ring that already fits. Enter either figure above.
- What is my US ring size in UK and EU?
- A US 6 is about UK L½ and EU 51.9; a US 7 is about UK N½ and EU 54.4; a US 8 is about UK P½ and EU 57.0; a US 9 is about UK R½ and EU 59.5. The EU (ISO) number is the inside circumference of the band in millimetres.
- How do millimetres relate to ring size?
- Ring size is based on the inside of the band. The inside diameter maps directly to a size — US 6 is about 16.5 mm and US 9 about 18.9 mm — and the circumference equals the diameter times about 3.14, which is the EU (ISO) size.
- What is the average ring size?
- The most common sizes are roughly US 6 to 6.5 for women and US 9 to 10 for men, though hand size varies widely. Fingers swell in heat and shrink in cold, so measure at a normal temperature.
- Should I size up for wide bands?
- Yes. Wide bands (about 6 mm or more) fit tighter than thin ones, so jewellers often suggest going up a quarter to a half size. If you are between sizes, choose the larger one.
- How accurate is this ring size converter?
- It uses the standard jewellers' conversion chart, so the US, UK, EU and Japan equivalents are accurate to the nearest standard size. The main source of error is your own measurement, so measure a few times and confirm with the jeweller before buying.