Measure Ring Size

Diameter calculator

Ring diameter calculator

You measured straight across the inside of a ring. Type that diameter in — in millimetres, centimetres or inches — and read its circumference and size equivalents instantly.

Ring diameter calculator

Decimal numbers are welcome. Use a dot for decimals, for example 16.5.

What you measured

Ring inner diameter

Changing the unit reinterprets the number you typed in the new unit — it does not convert it. Re-enter the number if you measured in a different unit.

Note: Measure straight across the inside of the ring, from inner edge to inner edge — not the outside, and not the band thickness. A ruler laid across the ring, read in millimetres, is more accurate than estimating from a photo.

Try an example

Highlight my country's size

Enter a measurement to see your size.

Formulas: circumference = π × diameter; diameter = circumference ÷ π. Matching uses your full, unrounded value; displayed diameter is rounded to 2 decimal places and circumference to 1 decimal place.

How to measure a ring's inner diameter

  1. Lay the ring flat

    On a flat surface, with good light. The ring should not roll or tilt while you measure.

  2. Measure edge to edge, inside only

    Place a ruler across the centre of the ring and read from the inner edge on one side to the inner edge on the other. Do not include the band thickness — that would measure the outside, which is several millimetres larger.

  3. Read in millimetres if you can

    Millimetres are the calculator's native unit and avoid a conversion step. If your ruler only shows inches, type the inches value and choose inches — the calculator converts exactly (1 inch = 25.4 mm).

Illustration of measuring a ring's inner diameter
Illustration only — not a sizing template. Measure a real ring's inner edge, then convert it in the calculator above.

Worked examples

16.51 mm → US 6

Circumference 51.9 mm, UK L (approx.), EU 52. Try it with the example button above.

17.32 mm → US 7

Circumference 54.4 mm, UK N (approx.), EU 54.

18.14 mm → US 8

Circumference 57.0 mm, UK P (approx.), EU 57.

Related guides

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