Three ways to describe the same ring
Britain sizes rings with letters, A upwards, with half sizes in between, so N½ sits between N and O. America uses numbers in quarter and half steps. Europe and the ISO standard skip the code altogether and just state the inside circumference in millimetres, which is the physical truth the other two systems are dancing around. That makes millimetres the safest common ground: measure once and you can read off any system.
The conversion between letters and numbers is a convention rather than exact maths, because the scales step by different amounts: a UK half letter is about 0.6 mm of circumference, a US quarter size about 0.64 mm. Charts from different jewellers therefore disagree by up to a quarter of a size, particularly at the extremes. This tool follows the most common jeweller convention, anchored on UK N½ = US 7 = 54.4 mm, and everything it shows should be treated as approximate. If a ring matters, confirm against the chart of the shop actually selling it.
The chart at a glance
The most-used stretch of the chart runs like this, inside circumference in millimetres:
- UK J½ = US 5 = 49.3 mm
- UK L½ = US 6 = 51.9 mm
- UK M½ = US 6½ = 53.1 mm
- UK N = US 6¾ = 53.8 mm
- UK N½ = US 7 = 54.4 mm
- UK P½ = US 8 = 57.0 mm
- UK R½ = US 9 = 59.5 mm
- UK T½ = US 10 = 62.1 mm
- UK V½ = US 11 = 64.6 mm
Most women's rings sold in the UK fall between J and P, most men's between P and W, but hands refuse to read averages, so measure the finger that will actually wear the ring. The full tool above covers F to Z in half sizes.
Worked examples
Measuring at home, honestly
Home measuring is fine for a rough size and treacherous for a precise one. String stretches and paper kinks, so use a thin, non-stretchy strip, wrap it snug but not tight at the base of the finger, and mark the overlap with a fine pen. Fingers change size with temperature and time of day, shrinking in the cold and swelling by evening, so measure warm hands at the end of the day and take the middle of two or three attempts. Check the size clears your knuckle, which on many hands is wider than the base of the finger, and remember that wide bands wear tighter: for anything 6 mm or chunkier, jewellers usually add half a size. The most reliable shortcut is a ring that already fits the right finger: measure its inside diameter across the widest point with a ruler and multiply by π (call it 3.14) to get the circumference, or trace the inside edge on paper if you are sizing someone secretly for a surprise. When the stakes are high, a jeweller will size your finger with proper gauges for free, and most plain metal bands can be resized a size or so either way if the surprise misses. For the other great sizing lottery, our shoe size converter works the same way.
Frequently asked questions
What is a UK N in US ring sizes?
UK N sits at about US 6¾, with an inside circumference of roughly 53.8 mm. It falls exactly between the common US half sizes, and UK N½ lines up with US 7. If the ring you want only comes in US half sizes, most people in an N would take the 7 for comfort rather than squeeze into a 6½.
How do I measure my ring size at home?
Wrap a thin strip of paper snugly around the base of the finger, mark where it overlaps, and measure the length in millimetres to get the circumference. Do it at the end of the day when fingers are warmest, repeat two or three times, and check the ring can pass over the knuckle. A jeweller will size you properly for free, which is worth it before an expensive purchase.
Why is the conversion only approximate?
The systems step by different amounts. A UK half letter is about 0.6 mm of circumference while a US quarter size is about 0.64 mm, so the scales slowly drift apart and published charts disagree by up to a quarter of a size, especially at the largest and smallest ends. Band width matters too, since a wide band wears tighter than a slim one.
What should I do if I am between two sizes?
Go up rather than down. The ring has to clear your knuckle, fingers swell in heat and through the day, and a slightly loose ring is far more wearable than a tight one. For chunky bands of 6 mm or more, jewellers usually suggest an extra half size. Most plain metal rings can also be resized a size or so either way later.
Do men and women use different ring size scales?
No, the scales are the same for everyone; only the typical range differs. Most women's rings in the UK fall between about J and P (roughly US 5 to 8) and most men's between about P and W (roughly US 8 to 11½). Hands vary hugely though, so measure rather than guess.