Ring sizes: converting US, UK and EU without guessing
A US size 7 is a UK size N and an EU size 54 — three totally different labelling systems for the same circle of metal.
The one real measurement underneath three labels
Every ring size system is ultimately describing the same physical thing: the internal circumference (or diameter) of the ring, usually in millimetres. Where the systems diverge is entirely in how that millimetre figure gets relabelled — the US uses an arbitrary numeric scale, the UK an alphabetical one, and the EU states the circumference itself, which makes EU sizing the most directly convertible of the three.
How each scale is built
US sizing runs numerically from about 3 to 13 in quarter-size steps, each step corresponding to roughly 0.4 mm of internal diameter — a scale invented by jewellers rather than derived from any external standard. UK sizing runs alphabetically from A to Z (then AA onward for larger sizes), with each letter also representing a fixed diameter step, historically tied to a scale of 1/12-mm circumference gradations dating to 19th-century British jewellery trade practice. EU sizing sidesteps both conventions and simply states the internal circumference in millimetres directly, so an EU size of 54 means a 54 mm internal circumference — no lookup table required to know the real-world dimension, unlike the other two.
Conversion table
| US | UK | EU (circumference mm) | Diameter (mm) |
|---|---|---|---|
| 5 | J½ | 49 | 15.7 |
| 6 | L½ | 51.5 | 16.5 |
| 7 | N | 54 | 17.3 |
| 8 | P½ | 57 | 18.1 |
| 9 | R½ | 59.5 | 19.0 |
| 10 | T½ | 62.5 | 19.8 |
Circumference and diameter relate the way they do for any circle: circumference equals diameter times π, so the EU figure and the millimetre diameter are themselves just another unit conversion, not two independent measurements.
Measuring a finger accurately
- Wrap a strip of paper or string snugly around the base of the finger and mark where it overlaps.
- Measure the length of that strip in millimetres — this is the circumference.
- Compare directly to the EU column above, or divide by π (3.14159) to get diameter and compare to the others.
- Measure at the end of the day, when fingers are at their largest, and avoid measuring in cold weather when fingers shrink slightly.
Why an existing ring is a better reference than a finger measurement
A ring that already fits well can be measured directly with a millimetre ruler across the internal diameter, avoiding the swelling and shrinkage that affects a live finger measurement by up to half a size across a day or a season. Jewellers' sizing sticks and printable EU-standard sizing rings both work from this same internal-diameter measurement, which is why the EU system, despite being the least commonly asked-about, is the one actually used to manufacture and verify sizing tools worldwide.
The takeaway
Convert through millimetres, not through the label — an EU circumference or a diameter in mm is the one figure that means the same thing everywhere, and the US number and UK letter are just regional relabellings of it.
Filed under Reference · Tags: ring size chart, us to uk ring size, ring size mm, finger circumference. Figures verified against the official definitions — see our editorial policy.