Ring Size Calculator

It uses the documented linear model usSize = (diameter - 11.63) / 0.8128, where each whole US size adds about 0.8128 mm of inside diameter. Real jewellers size against official steel mandrels, so the number is a close estimate rather than an exact grade. Treat a result like 5.99 as effectively size 6 and confirm before purchase.

Either works, because the tool converts between them using pi. Diameter is the straight distance across the inside of the ring, and circumference is the distance around the inside. Circumference is often easier to capture by wrapping a thin paper strip, while diameter is quicker with a ruler on a flat ring.

Yes. Use the mm and in toggle in the top right corner. When you switch units the tool converts your existing numbers at 25.4 mm per inch, and the diameter and circumference result cards show both mm and inches.

About 17.35 mm, which is close to a 54.5 mm inside circumference. Because sizes step by roughly 0.8128 mm of diameter, a half size is about 0.41 mm and a quarter size about 0.2 mm, so small measurement errors move the estimate.

The estimate is shown to two decimal places rather than snapped to a retail size, so you will see values like 6.24 or 7.03. Round to the nearest size your jeweller stocks, and if you land between two sizes, size up for comfort over a knuckle.

Fingers swell with heat, salt, exercise and time of day, and cold shrinks them. Measure two or three times at the end of a warm day and average the results so the band does not feel tight later.

No. The calculator outputs the US numeric scale only. You can still measure the diameter or circumference here and then read across to a UK letter or European (ISO) circumference on a standard conversion chart.

Yes. The calculation runs entirely in your browser with JavaScript, and nothing you type is uploaded or stored. Your measurements never leave your device.

It only accepts a positive diameter or circumference, so blank fields, zero and negative values are rejected. Enter a realistic measurement, for example a diameter between about 14 and 23 mm for most adult fingers.