Ring Size from Diameter Calculator

How it is calculated

The inner diameter gives the inner circumference, and the circumference is then converted to the US size scale with a linear formula.

C = π × d and US size = (C − 36.53) ÷ 2.55

d is the inner diameter of the ring in millimetres, measured across the inside of the band, and C is the inner circumference in millimetres. On this scale size 0 is a circumference of 36.53 mm and each full size adds 2.55 mm. Both results are shown to 1 decimal, and the size is not rounded to a half size. Only diameters from 12 to 25 mm are accepted; anything outside that range gives no result. The tool returns the US scale only, and UK or EU sizes use different scales.

Common mistakes

• Measuring the outer diameter, including the thickness of the band. Only the inside diameter, across the hole, gives the right size.
• Entering the circumference in the diameter field. A value like 54 is outside the accepted 12 to 25 mm range and gives no result.
• Entering the diameter in centimetres (1.73) or inches. The field is in millimetres.
• Reading the answer as a size you can order. Results like 7.9 must be rounded to a size the jeweller offers, and a snug or wide band may need a half size up.
• Using the US size for a UK or EU order. Those systems use different numbers for the same finger.