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Ring Size from Diameter Calculator

Measure the diameter, get your ring size.

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.

Worked example

A ring measures 17.3 mm across the inside. Circumference: π × 17.3 ≈ 54.35, shown as 54.3 mm in the result. Subtract the size-0 value: 54.35 − 36.53 = 17.82. Divide by 2.55 per size: 17.82 ÷ 2.55 ≈ 6.99. The tool shows a US ring size of 7.0. A ring of 18 mm would give 7.9, which you would round to the nearest available size.

Try this example →

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.

About this calculator

A ring's size ultimately comes down to the inner circumference of the band, which is derived directly from its inner diameter using the basic circle formula circumference = π × diameter. Because that circumference maps onto a standardized sizing scale, jewelers can convert a single diameter measurement — typically taken by slipping a ring onto a graduated mandrel or measuring an existing ring that fits — into a US ring size using published conversion tables.

Jewelers, ring resizing services, and online jewelry buyers use this diameter-to-size conversion constantly, especially for cross-checking a customer's self-measured ring against a printed ring sizer, converting between international sizing systems (US, UK, and EU sizes all use different scales for the same physical circumference), or verifying a size before an online purchase where trying the ring on isn't possible.

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