US against EU ring sizes: a scale against a measurement
These two systems are not two versions of the same thing. The EU/ISO number is a physical measurement, the inner circumference of the band in millimetres. The US number is a position on a scale with no unit attached. Almost every confusing thing about buying a ring abroad follows from that one asymmetry.
The EU number is the ring, written down
An EU 53 ring has an inner circumference of 53.1 mm. An EU 44 measures 44.2 mm and an EU 69 measures 69.7mm. The label is the measurement, rounded to the nearest whole millimetre, which is exactly what ISO 8653 specifies. You can check it with a strip of paper and a ruler, and if a jeweller's EU number disagrees with your measurement by more than half a millimetre, one of you is wrong and it is testable.
That is a genuinely unusual property in sizing. A shoe size does not tell you the length of the shoe. A dress size does not tell you anything at all. A ring size in the EU system tells you the circumference, and the rounding error is bounded and small.
The US number is a position, not a quantity
A US 6.5 is not 6.5 of anything. The scale starts at 0 and steps upward in quarters and halves, and the step happens to correspond to a fixed amount of finger, but the number itself carries no unit. This is why arithmetic on US ring sizes works for comparisons and fails for measurements: US 6.5 is bigger than US 3, but it is not 2.2 times anything.
The UK scale has the same property in a different notation. Letters instead of numbers, A at the bottom, running up through the alphabet with half sizes written as a letter and a half. No unit there either.
What one size is worth, at every point on the scale
This is the number the chart pages do not give you: not what a size is, but what the gap between two sizes is worth in millimetres. It decides whether going up a half size will actually fix a ring that feels tight, and it is the reason a jeweller talks in millimetres while a website talks in numbers.
| US step | Diameter gained (mm) | Circumference gained (mm) | EU/ISO equivalent |
|---|---|---|---|
| US 3 to 3.5 | 0.40 | 1.30 | 44 to 45 |
| US 3.5 to 4 | 0.40 | 1.30 | 45 to 47 |
| US 4 to 4.5 | 0.40 | 1.20 | 47 to 48 |
| US 4.5 to 5 | 0.40 | 1.30 | 48 to 49 |
| US 5 to 5.5 | 0.40 | 1.30 | 49 to 50 |
| US 5.5 to 6 | 0.40 | 1.30 | 50 to 52 |
| US 6 to 6.5 | 0.40 | 1.20 | 52 to 53 |
| US 6.5 to 7 | 0.40 | 1.30 | 53 to 54 |
| US 7 to 7.5 | 0.40 | 1.30 | 54 to 56 |
| US 7.5 to 8 | 0.40 | 1.30 | 56 to 57 |
| US 8 to 8.5 | 0.40 | 1.30 | 57 to 58 |
| US 8.5 to 9 | 0.50 | 1.20 | 58 to 59 |
| US 9 to 9.5 | 0.40 | 1.30 | 59 to 61 |
| US 9.5 to 10 | 0.40 | 1.30 | 61 to 62 |
| US 10 to 10.5 | 0.40 | 1.30 | 62 to 63 |
| US 10.5 to 11 | 0.40 | 1.20 | 63 to 64 |
| US 11 to 11.5 | 0.40 | 1.30 | 64 to 66 |
| US 11.5 to 12 | 0.40 | 1.30 | 66 to 67 |
| US 12 to 12.5 | 0.40 | 1.30 | 67 to 68 |
| US 12.5 to 13 | 0.40 | 1.20 | 68 to 69 |
Two things stand out. First, the step is close to constant: roughly 0.4 mm of diameter and about 1.3 mm of circumference per half size, right across the range. Ring sizing is linear in a way shoe sizing is not, where the step between neighbouring sizes swings between 0.4 and 0.5 cm depending on where you are on the chart. Second, the EU column does not always advance by the same amount, because it is a rounded millimetre value catching up with a scale that steps in fractions of a millimetre. That is why some EU numbers appear to skip.
What this means when you buy
If you are ordering from a country that uses the other system, convert once, from a millimetre measurement, rather than twice through two labels. Measuring the inside of a ring that already fits and reading the circumference straight into the EU column is the shortest and least lossy route to a correct size, whatever number ends up on the receipt.
Band width matters too, and no chart captures it. A wide band sits on more of the finger and reads tighter than a narrow band of the same measured size, which is typically worth a quarter to a half size. If you are between two rows and the ring is wide, take the larger one.
To measure properly, use the print-at-true-scale ring sizer or read how to measure ring size. For the full conversion table across every system, see the ring size chart, and for how the rounding is done see the methodology.
Common questions
Is a US ring size a measurement?
No. It is a position on an ordinal scale that starts at 0 and steps in quarters and halves. The number itself has no unit. The EU/ISO number does: it is the inner circumference in millimetres, so an EU 53 ring measures about 53 mm around the inside.
How many millimetres is one US ring size?
Roughly 0.8 mm of inner diameter, which is about 2.5 mm of circumference, and it is close to constant across the scale. That constancy is the useful part: unlike shoe sizes, a US half size is worth about the same amount of finger anywhere on the chart.
Which system should I measure in?
Millimetres, whichever label you end up buying in. A circumference in millimetres converts into every system without argument, while a US or UK label has to be translated and can be rounded twice on the way.
Why do UK ring sizes use letters?
Because the UK scale is alphabetical rather than numeric: A is the smallest and the sizes run up through the alphabet, with half sizes written as a letter and a half. It carries exactly the same information as the US number, in a different notation.
Last reviewed and updated on August 5, 2026. Conversion tables are checked against published international sizing standards.