Ring Size Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/10/2026
The French ring size corresponds exactly to the ring's inner circumference, in millimeters: circumference = diameter × π. An inner diameter of 16.5 mm gives a circumference of about 51.8 mm, rounded to French size 52.
Explanation
Unlike a shoe size, which follows a conventional scale with no direct link to a physical measurement, the French ring size system is directly defined by a geometric quantity: the ring's inner circumference, expressed in millimeters and rounded to the nearest whole number. A size 52 therefore corresponds, by definition, to a ring whose inner circumference measures about 52 mm. That is why this calculator starts from the diameter (easier to measure with a ruler on an existing ring than a circumference) and simply applies the geometric relationship circumference = diameter × π, rather than looking up a value in a table. This calculator deliberately limits itself to the French size: conversions to the American and British systems vary noticeably from one source to another, sometimes to the point of giving contradictory results for the same French size — for a purchase in a foreign system, it is better to use the jeweler's own ring sizer directly than an unreliable generic conversion. To convert a plain length, see our length conversion calculator, and for footwear, our shoe size conversion calculator.
Example: inner diameter of 16.5 mm
Inputs
Measured inner diameter: 16.5 mm.
Calculation
Circumference = 16.5 × π ≈ 51.8 mm. Rounded to the nearest whole number, this gives French size 52.
Result
An inner diameter of 16.5 mm corresponds to a French ring size of 52.
Frequently asked questions
How do you measure the diameter of an existing ring?
Lay the ring flat on a ruler and measure the distance between the two opposite inner edges of the ring, at the widest point. For more precision, measure several times and take the most frequent value, or use a ring sizer (often available free from a jeweler).
Why are conversions to US and UK sizes not offered?
Because the correspondence tables between the French size and the American or British systems vary noticeably from one source to another, sometimes to the point of giving different results for the same French size depending on the table used. Rather than display a conversion whose reliability is not guaranteed, this calculator limits itself to the verifiable geometric relationship (diameter, circumference, French size).
Does the size vary with the width of the band?
Yes, slightly: a very wide band (several millimeters) grips the finger more than a thin band at the same nominal inner diameter, which sometimes leads people to go half a size up for a wide ring. This calculator gives the theoretical size corresponding to the measured diameter, with no adjustment for band width.