acalculator

What is my ring size?

Measure the inside of a ring that fits, or wrap a strip of paper around your finger. The ring size calculator turns the measurement into a US ring size and an ISO size.

Your numbers

Units
I know the
US ring size
7

An inside diameter of 17.32 mm is US ring size 7 (ISO 54.4).

Exact US size
7
ISO size
54.4
Inside diameter (mm)mm
17.32
Inside diameter (in)in
0.682

US ring size: 7. An inside diameter of 17.32 mm is US ring size 7 (ISO 54.4).

How to calculate

Finds the US ring size and the ISO ring size from the inside diameter of a ring or the circumference of a finger, or the diameter and circumference of a US size.

Example with the default inputs (I know the Ring diameter, Inside diameter 17.32 mm): An inside diameter of 17.32 mm is US ring size 7 (ISO 54.4).

Method: Inside diameter (mm) = 11.63 + 0.8128 × US size; US size = (diameter − 11.63) ÷ 0.8128; ISO size = inside circumference (mm) = π × diameter.

  • US and Canadian sizes follow the common scale: size 0 is 11.63 mm (0.458 in) inside, and each whole size adds 0.032 in (0.8128 mm) to the diameter.
  • A finger circumference is taken as the ring’s inside circumference; the ISO size is that circumference in millimetres.
  • Jewelers’ mandrels and ring makers differ slightly; try a ring on before you buy.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. I know the Ring diameter, Inside diameter 17.32 mm gives US ring size 7, Exact US size 7.000492, ISO size 54.412385.Source: Ring size, Wikipedia: North American size to inside diameter d = 0.8128 × s + 11.63 mm (https://en.wikipedia.org/wiki/Ring_size, retrieved 2026-10-01); ISO 8653:2016 defines the size as the inside circumference in mm (https://www.iso.org/standard/66185.html)
  2. I know the US size, US size 7 gives Inside diameter (mm) 17.3196, ISO size 54.411128.Source: Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size): d = 0.8128 × 7 + 11.63 = 17.3196 mm
  3. I know the Finger circumference, Finger circumference 54.4 mm gives US ring size 7, Exact US size 6.995642.Source: Ring size, Wikipedia (https://en.wikipedia.org/wiki/Ring_size); ISO 8653:2016 (size = inside circumference in mm)

How it works

Pick what you know: a ring's inside diameter, a finger's circumference, or a US size. With d the inside diameter in millimetres and s the US size:

  • d = 11.63 + 0.8128 × s
  • s = (d − 11.63) ÷ 0.8128
  • From a circumference C: d = C ÷ π
  • ISO size = inside circumference in millimetres = π × d

The calculator shows:

  • US ring size: s rounded to the nearest quarter size (0.25), with an exact eighth (for example 7.125) rounded up to 7.25. Shown with 2 decimals.
  • Exact US size: s to 3 decimals.
  • ISO size: π × d to 1 decimal.
  • Inside diameter: d in millimetres to 2 decimals and in inches (d ÷ 25.4) to 3 decimals.

Displayed values are rounded half up. A typed diameter or US size is read as the exact decimal you typed (in the unit you typed it), so the size from a diameter is exact before rounding; a circumference goes through π, so that path runs in floating point.

Rules

  • Lengths are from 0.001 m (1 mm) to 0.2 m (200 mm); 1 in = 25.4 mm and 1 cm = 10 mm exactly.
  • US sizes run from 0 to 16. A measurement outside that range has no answer: "This is outside US sizes 0 to 16 (an inside diameter of 11.63 mm to 24.6348 mm, a circumference of about 36.54 mm to 77.39 mm)."

Worked examples by hand

A ring 17.32 mm across. s = (17.32 − 11.63) ÷ 0.8128 = 7.0005, so US size 7. ISO size = π × 17.32 = 54.4.

US size 7. d = 11.63 + 7 × 0.8128 = 17.3196 mm (0.682 in). ISO size = π × 17.3196 = 54.4.

A finger 54.4 mm around. d = 54.4 ÷ π = 17.316 mm. s = (17.316 − 11.63) ÷ 0.8128 = 6.996, so US size 7.

Other questions people ask

How do I find my ring size at home?

Measure the inside diameter of a ring that fits the finger, edge to edge, in millimetres. Or wrap a thin strip of paper around the finger where the ring sits, mark where it overlaps, and measure that length: the circumference. Type either one.

How do US ring sizes work?

Size 0 has an inside diameter of 11.63 mm (0.458 in), and each whole size adds 0.032 in (0.8128 mm). So size 7 is 11.63 + 7 × 0.8128 = 17.32 mm across, and half and quarter sizes sit in between.

What is an ISO ring size?

ISO 8653 gives a ring size as the inside circumference in millimetres. A US size 7 ring (17.32 mm across) has an inside circumference of π × 17.32 = 54.4 mm, so it is ISO size 54.4, often marked 54.

What ring size is a 54 mm circumference?

54 mm ÷ π = 17.19 mm across, which is US size (17.19 − 11.63) ÷ 0.8128 = 6.84, about 6 3/4.

Should I round up or down between sizes?

The calculator shows the nearest quarter size. Fingers swell in heat and later in the day, and wide bands fit tighter, so many people pick the next quarter size up when in doubt.

Are Canadian ring sizes the same as US sizes?

Yes. The US and Canada use the same number scale. The UK and Australia use letters, and much of Europe uses the ISO circumference in millimetres.