Ring size converter

Use the converter below to move between the six sizing systems Ring Sizer supports. It shows every equivalent at once, so you never have to chain two conversions together and compound the rounding error that comes with each step.

How does the conversion actually work?

There is no lookup magic involved. Every ring size system is a label attached to a physical measurement, so converting between them means going back to the millimetres and re-labelling.

The process has three steps. Take the size you have. Look up its inner circumference in millimetres, which for European sizes is simply the number itself. Then read off whatever label the target system attaches to that circumference.

Because the US scale is linear, adding 2.5535 mm of circumference per size, and the ISO scale is the circumference itself, those two convert exactly with no approximation at all. Japanese and Chinese scales are integer-stepped and land close to, but not always precisely on, a US half size, which is why some published charts disagree by one step at the extremes of the range.

Conversion quick reference

The full table, in half-size steps from a US 3 to a US 13, is on the ring size chart.

Why do UK letters not line up with US numbers?

Because the two scales were standardised separately and step at different intervals.

One full US size is about 2.55 mm of circumference. One full UK letter is about 1.27 mm, which is almost exactly half. So a single US size spans two UK letters, and most US whole sizes land on a UK half letter rather than a whole one. A US 6 is a UK L½, not L.

Dropping the half puts you an eighth of a size out per step, and the error compounds as you move along the scale. It is the most common mistake in this particular conversion, which is why the US to UK guide covers that pair on its own page.

Why do European sizes look so different?

Because they are measurements rather than sizes. An EU or ISO size is the ring’s inner circumference in millimetres, stated plainly.

A seller quoting “size 54” means 54 mm around the inside, which works out at a 17.2 mm diameter and a US 7. If a European size looks like it would fit a bracelet, you have read a circumference as though it were a diameter.

This is the only system where the number tells you something true without a chart, which is why people who buy across markets tend to end up preferring it. It is also codified as ISO 8653, which makes it the closest thing the industry has to a neutral standard.

Are Japanese and Chinese sizes the same as US ones?

No, and the resemblance is a trap. Japanese size 13 is a US 6½, not a US 13.

The two scales share digits and nothing else. Japan uses its own integer progression fixed by the Japanese jewellery trade, and most conversion charts treat Chinese sizes as sharing that scale. Both run from roughly 1 to 27, which overlaps almost exactly with the useful range of US sizes and makes the confusion easy.

If somebody quotes you a bare number in the teens with no country attached, ask before you order. The difference between a Japanese 13 and a US 13 is more than six sizes.

What if your system is not on any chart?

Some independent makers use in-house sizing, and plenty of vintage rings predate standardisation altogether. In both cases, do the thing that always works.

Measure the inner diameter in millimetres and send that number, with the word “diameter” attached so there is no ambiguity about which quantity you mean. Millimetres are the universal language of ring sizing, and a jeweller who cannot work from them is not a jeweller you want cutting metal for you.

If you are buying vintage and cannot measure the ring at all, ask the seller for the inner diameter rather than a size. Sellers frequently quote a size they have converted themselves from a measurement, and asking for the original measurement removes their conversion from the chain.

What if you do not have a size to convert yet?

A converter only helps once you have a starting number, and getting that number is the harder half of the problem.

If you own a ring that fits the correct finger, measure its inner diameter and enter that. It is the most accurate route available at home, because you are measuring a rigid object rather than a finger that changes size through the day.

If you do not, measure the finger itself at the base, in the evening, at room temperature, with warm hands. Expect about half a size of uncertainty and take three readings rather than one.

Either way, enter the millimetre figure into the converter rather than a size you have already converted yourself. Every conversion rounds, and starting from the raw measurement means only one rounding step stands between your finger and the number you order.

Which other tools are worth knowing about?

The converter above handles the arithmetic. Four other pages cover the parts of the problem it cannot.

The full international size chart lists every size in every system in half-size steps, which is the page to use when you want to see the whole range at once rather than convert a single value.

mm to ring size works in the other direction, turning a raw measurement into a size, and explains how to tell whether the number you have is a diameter or a circumference.

US to UK ring size covers the single most-needed conversion pair in detail, including the half-letter rule that catches most people out.

And how to measure ring size explains how to produce a trustworthy number in the first place, which is the step everything else depends on.