Ring Size Calculator

This calculator converts a finger measurement or the inside diameter of a ring that already fits into US/Canadian, EU/ISO and approximate UK/Australian sizes. Enter millimeters; if your tape is in inches, convert with the length converter first (1 in = 25.4 mm).

Use it when buying a ring online, checking a size for a gift using a ring the person already wears, or comparing sizes from a jeweler in another country. Measuring an existing ring's inside diameter is usually more accurate than wrapping paper around a finger, because paper stretches and is hard to read to the nearest millimeter.

The US scale is a fixed formula, so its conversion is exact. UK letter sizes come from a separate British standard and the calculator gives the nearest letter, which can differ from a jeweler's chart by half a size. Wide bands, knuckles larger than the base of the finger and cold hands all affect fit, so treat the result as a starting point.

The US and Canadian scale starts at size 0 = 0.458 in (11.63 mm) inside diameter and adds 0.032 in of diameter per size, which is about 2.55 mm of circumference. ISO 8653 defines a ring size as the inside circumference in millimeters, which is what most of continental Europe uses. UK and Australian sizes use letters A–Z defined in British Standard BS 6820; half-letter steps are about 1.25 mm of circumference. To switch between diameter and circumference for other round objects, use the circle calculator.

US sizes and millimeter values below are calculated from the US formula. UK letters are the usual equivalents on jewelers' charts, which can vary by half a letter between retailers.

• Use non-stretch material: a thin strip of paper or string, laid flat, not twisted. A flexible tape measure is better still.
• Measure at the base, then the knuckle: the ring must pass the knuckle, so if that measurement is larger, size between the two.
• Warm hands, late in the day: fingers are smallest when cold and in the morning, and swell with heat, exercise, salty food and alcohol. Measure two or three times on different days.
• Dominant hand is usually larger: by up to half a size, so measure the actual finger the ring will go on.
• Read to the nearest millimeter: a 1 mm error in circumference is about 0.4 of a US size. A 1 mm error in diameter is over a full size, so measure diameters carefully.

A wide band covers more of the finger and feels tighter than a thin one of the same size. Many jewelers suggest going up about half a size for bands of 6 mm or wider. Comfort-fit bands, which are rounded on the inside, feel slightly looser, so some people go down a quarter or half size. For an engagement ring meant as a surprise, err slightly large: resizing down is usually easier and cheaper than stretching a ring up.