Cm to Ring Size Converter
Turn a string-and-ruler measurement into a ring size. Enter the measurement in centimeters, tell the converter whether it is the finger's circumference or a ring's inner diameter, and get the closest US half size, the EU/ISO size, and the exact diameter in millimeters.
Example: with Measurement (cm) 5.4 · This measurement is the Circumference (string around the finger) → US ring size: US 7.
- Exact US size number6.84
- EU / ISO sizeEU 54 (54.0 mm around)
- Inner diameter17.19 mm
Computed by the calculator below using its default values. Change any input to see your own numbers.
US size = (inner diameter in mm − 11.63) ÷ 0.8128 — each full size adds 0.8128 mm (0.032 in) of diameter. EU/ISO size is simply the circumference in mm.
From string to size
Wrap a strip of paper or thin string snugly around the base of the finger, mark where it overlaps, and measure the length in centimeters — that is the circumference. Dividing by pi gives the diameter, and US sizes are just a linear scale on diameter: size 0 starts at 11.63 mm and each full size adds 0.8128 mm. A 5.4 cm circumference works out to a 17.19 mm diameter, an exact size of 6.84, which a jeweler would round to US 7.
European sizing skips the arithmetic entirely: the EU/ISO size is the circumference in millimeters, so 5.4 cm is simply EU 54. That is why EU sizes look like big numbers and US sizes look small — same finger, different convention.
How it’s calculated
Diameter = circumference ÷ π (or taken directly in diameter mode). US size = (diameter in mm − 11.63) ÷ 0.8128, rounded to the nearest half size — the standard US scale steps 0.8128 mm (0.032 in) of diameter per size. EU/ISO size = inner circumference in mm (ISO 8653). 1 cm = 10 mm.
A string or paper strip stretches and flattens slightly, so it tends to read a half size large — a jeweler's mandrel or sizer set is the reliable check before buying an expensive ring.
Circumference in cm to US ring size
| Circumference | Inner diameter | US size |
|---|---|---|
| 4.4 cm | 14.01 mm | US 3 |
| 4.7 cm | 14.96 mm | US 4 |
| 4.9 cm | 15.60 mm | US 5 |
| 5.2 cm | 16.55 mm | US 6 |
| 5.4 cm | 17.19 mm | US 7 |
| 5.7 cm | 18.14 mm | US 8 |
| 6.0 cm | 19.10 mm | US 9 |
| 6.2 cm | 19.74 mm | US 10 |
Computed from US size = (diameter − 11.63 mm) ÷ 0.8128 mm; matches standard US jeweler charts.
Common mistakes
- Measuring a cold finger — fingers shrink noticeably in the cold; measure at the end of a warm day when they run largest.
- Pulling the string tight: it must sit snug but still slide over the knuckle, or the ring never will.
- Confusing circumference with diameter — a 5.4 cm circumference is US 7, but 5.4 cm of diameter is off every chart.
- Ignoring band width: comfort-fit and 6 mm+ wide bands wear about a half size smaller than thin bands.
Frequently asked questions
What is the cm to ring size formula?
Divide the circumference in mm by π to get the diameter, then US size = (diameter − 11.63) ÷ 0.8128. Example: 5.4 cm = 54 mm around = 17.19 mm across = US size 7.
Is the EU size just millimeters?
Yes — the EU/ISO size equals the inner circumference in millimeters, so a 5.4 cm (54 mm) measurement is EU 54. No lookup table needed.
What if I land between two sizes?
Round up. A slightly loose ring can be sized down or worn with a guard; a too-tight ring will not go over the knuckle at all.
What are the most common ring sizes?
US jewelers commonly cite 6 to 7 for women and 9 to 10 for men. That is only a starting point — dominant hands and knuckle shape move individual fingers a full size either way.