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Ring size calculator: from finger measurement to US and French sizes

Find your ring size from the circumference of your finger or the inside diameter of a ring that fits, in US, French and Italian sizes.

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US and Canada ring size

7.00

Wide bands (over 6 mm) usually need a quarter to half size larger

French size (ISO, circumference in mm)

54

Italian, Spanish and Swiss size
14
Inside diameter
17.3 mm
Inside circumference
54.4 mm

Finding the right size without a jeweler

Rings are sized by their inside dimensions, and the systems differ around the world. In the United States and Canada, sizes run from about 3 to 13 in quarter steps; in France and most of continental Europe, the size is simply the inside circumference in millimeters. This calculator converts a measurement of your finger, or of a ring that already fits, into both.

Measuring at home

With a string or paper strip:

  1. Wrap a thin strip of paper or a non-stretchy string snugly around the base of the finger.
  2. Mark where the end overlaps, then lay it flat and measure the length in inches or millimeters.
  3. Make sure a ring of that size will slide over the knuckle; if the knuckle is larger, measure it too and size between the two.

With a ring that fits:

  1. Lay the ring on a ruler and measure the inside diameter from edge to edge, not including the band itself.
  2. Measure across the widest point. A millimeter ruler gives the best precision.

How the sizes are calculated

  • US and Canada: inside diameter in mm = 11.63 + 0.8128 × size. Each whole size adds about 2.55 mm of circumference.
  • France (ISO 8653): size = inside circumference in mm, rounded to the nearest whole number.
  • Italy, Spain and Switzerland: circumference in mm minus 40.

Worked example

A string around your ring finger measures 2.14 inches (54.4 mm).

  • Inside diameter: 54.4 ÷ π = 17.3 mm.
  • US size: (17.3 − 11.63) ÷ 0.8128 = 6.98, so size 7.
  • French size: 54.
  • Italian size: 14.

Tips for an accurate fit

  • Measure at the end of the day and at room temperature. Fingers swell in heat and shrink in the cold.
  • Measure two or three times and use the average.
  • Wide bands, over about 6 mm, fit more tightly. Go up a quarter to half size.
  • Your dominant hand is often slightly larger.
  • Sizes between whole numbers matter: a half size is only about 1.3 mm of circumference, but it is the difference between a ring that spins and one that will not pass the knuckle.
  • If the ring is a surprise, borrow one the person wears on the same finger and measure its inside diameter.

Frequently asked questions

How do I measure my ring size at home?

Wrap a strip of paper or a non-stretchy string around the base of the finger, mark where it overlaps, and measure the length with a ruler. Measure at the end of the day, when fingers are largest, and make sure the ring will pass over the knuckle.

How are US ring sizes defined?

Each whole size adds 0.032 inch (0.8128 mm) to the inside diameter, starting from 0.458 inch (11.63 mm) for size 0. A US size 7 has an inside diameter of about 17.3 mm.

What is the French ring size?

French and ISO sizes are simply the inside circumference in millimeters. A finger of 54 mm takes a size 54, which is about a US 6¾ to 7. Italian, Spanish and Swiss sizes subtract 40 from that number.

Should I size up for a wide band?

Usually yes. Bands wider than about 6 mm fit more snugly, so jewelers often recommend a quarter to half size larger. Comfort-fit bands, rounded on the inside, may need the usual size.

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Results are estimates provided for information purposes. They do not replace professional advice.

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