Ring Size Converter

How ring size conversion works

Every ring size system measures the same thing, the inside of the band, but each one counts it differently. US and Canadian sizes are a numbered scale in which each full size adds 0.8128 mm (0.032 in) to the inner diameter. EU sizes, based on ISO 8653, are simply the inner circumference in millimeters. UK and Australian sizes use letters from A to Z, with half letters in between, and each full letter is close to a US half size. Shoes have the same country-by-country problem, which the US to UK shoe size converter and the rest of the size converters handle.

Step by step

1. Choose your US or Canadian size in the converter, including half sizes.
2. The inner diameter comes from 11.63 mm plus 0.8128 mm for every size.
3. The circumference is the diameter multiplied by π (3.1416).
4. Round the circumference to the nearest millimeter for the EU size, and read the UK letter from the chart.

Worked example: for a US 7, the diameter is 11.63 + 0.8128 × 7 = 17.32 mm, shown as 17.3 mm. The circumference is π × 17.32 = 54.41 mm, so the EU size is 54 and the UK size is N½. Moving up half a US size adds about 0.41 mm of diameter and 1.28 mm of circumference, which is why UK letters, at roughly 1.25 mm of circumference each, track US half sizes so closely. For measurements taken in inches, the inches to cm converter gets you to millimeters.

Sizes vary slightly between jewelers, mandrels and countries, and a wide band fits tighter than a thin one of the same size. Treat the chart as a guide, and for an important purchase such as an engagement ring, have your finger sized by a jeweler.

How to measure your ring size at home

From a ring that already fits

1. Choose a ring that fits the same finger comfortably.
2. Lay it on a ruler and measure the inside diameter, edge to edge across the widest point, in millimeters.
3. Find the nearest diameter in the chart. A 17.3 mm inner diameter, for example, is a US 7.

With a strip of paper or string

1. Wrap a thin strip of paper or non-stretch string around the base of the finger, snug but not tight.
2. Mark where it overlaps, then lay it flat and measure the length in millimeters. That is the inner circumference.
3. Look it up in the circumference table above, or divide by π for the diameter.
4. Slide the loop over your knuckle to make sure a ring of that size will actually go on.

Paper and string can stretch or be pulled too tight, so repeat the measurement two or three times. Printable ring sizers only work if printed at exactly 100% scale; check the printed ruler against a real one before you rely on it.

Should I size up or down between two ring sizes?

Size up when in doubt: a slightly loose ring can be snugged with a sizing bead or spring insert, while one that will not pass your knuckle cannot be worn at all. These checks help you decide:

• Measure at the right time. Fingers are larger at the end of the day and in warm weather, and smaller when cold. Measure at room temperature, late in the day.
• Wide bands fit tighter. A band wider than about 6 mm covers more of the finger, so many jewelers suggest going up about half a size.
• Mind the knuckle. If your knuckle is much larger than the base of the finger, pick a size that clears the knuckle and let a jeweler add a bead if the band then turns on the finger.
• Each hand is different. The fingers on your dominant hand are often slightly larger, so measure the exact finger that will wear the ring.
• Resizing has limits. Many gold, silver and platinum rings can be resized by a jeweler, but tungsten, ceramic, some titanium rings and bands with stones all the way around usually cannot, so accurate sizing matters most for those.

For shoes, the same measure-first logic applies: the US to EU shoe size converter lists the foot length next to every size.