UK ring size O in US and European sizes
UK ring size O is US size 7.25 and European (ISO) size 55.1. The inside diameter is 17.55 mm and the circumference 55.13 mm.
The UK, Ireland, Australia and New Zealand share this letter scale. Europe uses the circumference in millimeters directly, which is why the European size and the circumference are the same figure.
How to check this is your size
The surest way is to measure a ring that already fits the right finger. Lay it flat and measure straight across the inside of the band. If it reads about 17.55 mm (0.691 in), you are a UK O.
If you have no ring to hand, wrap a strip of paper around the base of the finger, mark where it overlaps, and measure the length. About 55.13 mm (2.17 in) means UK O. Measure at the end of the day, when fingers are at their largest — a ring sized on a cold morning will feel tight by evening.
UK O is in the most common range for women's rings; US 6 to 7 covers roughly half of all women.
UK O and its neighbors
| US | UK / AU | Europe | India / Japan | Diameter | Circumference |
|---|---|---|---|---|---|
| 5.75 | L | 51.2 | — | 16.3 mm | 51.21 mm |
| 6 | L½ | 51.9 | 11 | 16.51 mm | 51.87 mm |
| 6.25 | M | 52.5 | — | 16.71 mm | 52.5 mm |
| 6.5 | M½ | 53.2 | 12 | 16.92 mm | 53.16 mm |
| 6.75 | N | 53.8 | 13 | 17.13 mm | 53.82 mm |
| 7 | N½ | 54.5 | 14 | 17.35 mm | 54.51 mm |
| 7.25 | O | 55.1 | — | 17.55 mm | 55.13 mm |
| 7.5 | O½ | 55.8 | 15 | 17.75 mm | 55.76 mm |
| 7.75 | P | 56.5 | — | 17.97 mm | 56.45 mm |
| 8 | P½ | 57.1 | 16 | 18.19 mm | 57.15 mm |
| 8.25 | Q | 57.8 | — | 18.39 mm | 57.77 mm |
| 8.5 | Q½ | 58.2 | 17 | 18.53 mm | 58.21 mm |
| 8.75 | R | 59.1 | — | 18.8 mm | 59.06 mm |
One half size is a very small step: going up to 7.75 (UK P) adds 1.32 mm of circumference, and down to 6.75 (UK N) removes 1.31 mm. Quarter sizes exist and a good jeweler will make one; if you sit between two half sizes, choose the smaller for a narrow band and the larger for a wide one.
The full chart covers every size from US 3 to US 16 in all five systems.
Adjusting for the band
A wide band touches more of the finger and feels tighter at the same measurement, and a comfort-fit band (domed on the inside) feels looser. For a finger that measures 55.13 mm:
| Band | Order | ||
|---|---|---|---|
| Narrow band, under 4 mm | US 7.25 | UK O | EU 55.1 |
| Medium band, 4–6 mm | US 7.5 | UK O½ | EU 55.8 |
| Wide band, 6 mm and over | US 7.75 | UK P | EU 56.5 |
| Comfort-fit (domed inside) | US 7 | UK N½ | EU 54.5 |
What a plain band in this size weighs
Useful if you are buying by weight or checking a quote. A solid band 1.6 mm thick at UK O, with the metal alone valued at today's spot price:
| Band | Weight | Metal value today |
|---|---|---|
| 2 mm 14K | 2.52 g | $184 |
| 2 mm 18K | 2.98 g | $279 |
| 2 mm Platinum | 3.87 g | $170 |
| 4 mm 14K | 5.04 g | $367 |
| 4 mm 18K | 5.97 g | $558 |
| 4 mm Platinum | 7.74 g | $340 |
| 6 mm 14K | 7.57 g | $552 |
| 6 mm 18K | 8.95 g | $837 |
| 6 mm Platinum | 11.61 g | $510 |
A jeweler's price is well above the metal value — it carries the making, the finish and the margin — but the metal figure is the floor the piece can never fall below, and the number a scrap buyer starts from.
Measure it on your screen
Lay a ring against circles drawn true to life, calibrated with any bank card.
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