Ring Size from Circumference Calculator

How it is calculated

The finger circumference is converted to the US size scale with a linear formula, and the diameter follows from the circle relation.

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

C is the circumference in millimetres, for example measured with a strip of paper or string around the base of the finger. On this scale size 0 is 36.53 mm and each full size adds 2.55 mm. The result also shows the inner diameter d of a ring that fits, in millimetres. Both values are shown to 1 decimal and the size is not rounded to a half size. The accepted range is 38 to 78 mm; values outside it give no result. The tool shows the US scale only, and UK or EU sizes are numbered differently.

Common mistakes

• Entering the finger diameter in the circumference field. A value like 18 is below the accepted 38 mm minimum and gives no result.
• Measuring in inches or centimetres. The field expects millimetres, so 5.7 cm must be typed as 57.
• Measuring with a thick or stretchy strip, or pulling it tight. Use a thin non-stretch strip and read the mark where it overlaps, snug but not squeezing.
• Measuring at the wrong time. Fingers are usually smaller when cold, so measure when your hand is at normal temperature.
• Ignoring the width of the ring. A wide band feels tighter than a thin one of the same size, so go slightly larger for a wide ring.