The systems are not variations on a theme; they are built on different principles. Understanding that is what stops a conversion going wrong.
Enter a millimetre measurement to get your size in every system at once.
Your size
There is no single global ring size. A "size 7" means nothing until you know which country's system it belongs to, and the systems are not variations on a theme — they are built on different principles.
This page explains how each one works. If you just want the numbers, the full conversion chart has them; this is the page for understanding why the conversion is awkward, which is what stops you making an expensive mistake.
Underneath every scale are two physical measurements that never change: the inside diameter of the band, and the inside circumference. They are the same fact expressed two ways, related by pi.
When a size label is ambiguous, ask for the millimetres. Inside diameter or inside circumference are physical measurements, so they mean the same thing to any jeweller anywhere, and no conversion can go wrong.
| US / Canada | UK / AU / IE | EU / ISO | Inside circumference | Inside diameter |
|---|---|---|---|---|
| 3 | F | 44 | 44.2 mm | 14.1 mm |
| 3.5 | G | 45–46 | 45.5 mm | 14.5 mm |
| 4 | H | 47 | 46.8 mm | 14.9 mm |
| 4.5 | I | 48 | 48.0 mm | 15.3 mm |
| 5 | J½ | 49 | 49.3 mm | 15.7 mm |
| 5.5 | K½ | 50–51 | 50.6 mm | 16.1 mm |
| 6 | L½ | 52 | 51.9 mm | 16.5 mm |
| 6.5 | M½ | 53 | 53.1 mm | 16.9 mm |
| 7 | N½ | 54 | 54.4 mm | 17.3 mm |
| 7.5 | O½ | 56 | 55.7 mm | 17.7 mm |
| 8 | P½ | 57 | 57.0 mm | 18.1 mm |
| 8.5 | Q½ | 58 | 58.2 mm | 18.5 mm |
| 9 | R½ | 59–60 | 59.5 mm | 18.9 mm |
| 9.5 | S½ | 61 | 60.8 mm | 19.4 mm |
| 10 | T½ | 62 | 62.1 mm | 19.8 mm |
| 10.5 | U½ | 63 | 63.4 mm | 20.2 mm |
| 11 | V½ | 64–65 | 64.6 mm | 20.6 mm |
| 11.5 | W½ | 66 | 65.9 mm | 21.0 mm |
| 12 | X½ | 67 | 67.2 mm | 21.4 mm |
| 12.5 | Y½ | 68–69 | 68.5 mm | 21.8 mm |
| 13 | Z½ | 70 | 69.7 mm | 22.2 mm |
US sizes run from about 3 to 13 for adults, in half and sometimes quarter steps. The numbers are labels rather than measurements: a size 6 is not six of anything. The scale derives historically from the diameters of the mandrels American jewellers used, and each full size step works out at about 2.5 mm of circumference.
Canada uses the same scale. So do many international online retailers by default, which is why US sizing is the most common thing to be quoted if a site does not say.
Letters from A upward, with half sizes written as N½. Each letter step is about 1.25 mm of circumference, so a single US size spans roughly two UK letters. The two scales also start in different places, which is why US whole sizes tend to land on UK half-letters. The letter scale in detail
Under ISO 8653, the size is the inside circumference in millimetres. EU 54 means 54 mm around the inside. This is the only major system where you can measure your size directly with a paper strip and a ruler, with no lookup at all. Why EU sizing is the simplest
A variant of the ISO approach. Subtract 40 from the circumference: an ISO 54 becomes a 14. If sizes from a Southern European or Swiss retailer look implausibly small, this is almost certainly why. Add 40 to convert back.
Japan uses its own numeric scale, generally running from 1 to about 27. It does not correspond to the US numbers or to a millimetre measurement, so it needs its own lookup. Roughly, a Japanese 13 is near a US 6.5 and a Japanese 14 near a US 7, but published Japanese conversions vary a little between sources.
China and much of East Asia often use the Japanese scale or the ISO circumference. Because there is more variation here than in the Western systems, this is a case where quoting millimetres is worth the extra message.
Indian ring sizing uses numbers, commonly running to around 30, and again does not map onto the US or ISO systems arithmetically. Indian jewellers will normally convert from a diameter or circumference figure without difficulty, so give them that.
If you shop internationally with any regularity, four anchor points cover most situations:
The single most expensive mistake. A UK retailer listing "size 7" may mean the US scale for international customers, or may not. A Japanese "14" is not a European 14. If a listing gives a number with no system named, ask before ordering.
Converting US to EU and then EU to UK compounds two rounding errors and can leave you a half size out. Convert once, from the millimetre measurement, to whatever you need.
There is not one. The scales are historically unrelated. Any "formula" you find is a curve fitted to a lookup table, and it will drift at the ends of the range.
They differ, by up to roughly a quarter of a size between manufacturers. For a fashion ring this is immaterial. For an engagement ring it is worth checking the specific retailer's own chart rather than a general one.
The millimetre figures are consistent and checked, but the size labels attached to them are convention rather than enforced standard. Published charts vary between manufacturers by up to roughly a quarter of a size. When precision matters, quote the inside circumference in millimetres and use the retailer's own chart rather than a general one.
No. The two scales are historically unrelated and were never designed to map onto each other, so a lookup table is always required. The closest useful rule of thumb is that one US size is about two UK letters, which gets you near the right answer from a known anchor.
By number of countries, the ISO circumference system used across continental Europe is the most straightforward and the most widely followed standard. By volume of online retail, US sizing is what you will most often encounter, because many international retailers default to it.
It depends entirely on the system. From an Italian, Spanish or Swiss retailer it likely means circumference minus 40, so ISO 54 — a US 7. From a Japanese retailer it is roughly a US 7 on their own separate scale. A US 14 would be an unusually large ring. Ask which system is meant.
Yes, and it is the most reliable way. Every system is a label for an inside diameter or circumference, so converting once from the millimetre figure avoids the compounding rounding errors you get from converting US to EU to UK in sequence.
Broadly, but not exactly. There is no enforced global standard for the US and UK scales, and published charts vary by up to roughly a quarter of a size. For anything expensive or hard to resize, use the retailer's own chart and confirm before ordering.
The letter scale, its irregular step, and sizes above Z.
US to EU in depthISO sizing, half sizes, and the Italian minus-40 variant.
The full conversion chartEvery size from US 3 to 13 with all systems side by side.
Get an unambiguous measurementMillimetres mean the same thing to any jeweller, anywhere.