CM to MM Ring Size Converter — Finger Circumference to Ring Size
Enter your finger circumference in centimeters, get the millimeter equivalent, and instantly see your EU/ISO, US and UK ring size.
cm
EU/ISO ring size: 52
- 1
Finger circumference
5.2 - 2
Circumference in mm
5.2 × 10 = 521 cm = 10 mm; EU/ISO size equals this value rounded.
How does this calculator work?
Multiply cm by 10 to get mm. EU ring size = mm rounded. US size = (mm − 36.5) ÷ 2.617. UK: find nearest letter (e.g., 52 mm ≈ UK N). Example: 5.2 cm → 52 mm → EU 52, US ≈ 5.9, UK ≈ N.
Formula
How this is calculated
Ring sizes are based on the inner circumference (or diameter) of a ring. The EU/ISO standard used across continental Europe is the most transparent: the size number equals the inner circumference in millimeters, rounded to the nearest whole number — so EU 52 means 52 mm inner circumference, giving a diameter of 52 ÷ π ≈ 16.56 mm. Converting cm to mm is the first step: multiply by 10.
US ring sizes use a different scale. The widely cited formula is US size = (circumference_mm − 36.508) ÷ 2.617, where each full US size equals approximately 2.617 mm of circumference. US half-sizes (e.g., 6.5) are common — round the result to one decimal place. UK sizes use a letter system (A through Z+3 and beyond) derived from a fixed reference table; this calculator finds the nearest letter for your circumference using standard published values.
To measure at home: cut a narrow strip of paper, wrap it snugly around the base of your finger (use the knuckle if it is wider than the base), mark where it meets, and measure the strip with a ruler. That length in mm is your circumference. Add about 1 mm for a looser fit.
Frequently asked questions
Wrap a paper strip around your finger, mark the overlap, and measure the strip in cm. Multiply by 10 for mm. That mm value is approximately your EU/ISO ring size.
Women's ring circumferences average around 52–55 mm (EU 52–55, US 6–7). Men's average around 60–68 mm (EU 60–68, US 9–11.5). Always measure your own finger rather than relying on averages.
Yes — EU/ISO ring size equals the inner circumference in millimeters (rounded). EU 52 = 52 mm circumference = 16.56 mm inner diameter.
Also known as
TG we-Calculate Editorial Team. (2026). CM to MM Ring Size Converter — Finger Circumference to Ring Size [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/cm-to-mm-ring-size-converter
TG we-Calculate Editorial Team. "CM to MM Ring Size Converter — Finger Circumference to Ring Size." TG we-Calculate. 2026. https://we-calculate.com/calculator/cm-to-mm-ring-size-converter.
TG we-Calculate Editorial Team, "CM to MM Ring Size Converter — Finger Circumference to Ring Size," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/cm-to-mm-ring-size-converter
@misc{wecalculate_cm_to_mm_ring_size_converter, title = {CM to MM Ring Size Converter — Finger Circumference to Ring Size}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/cm-to-mm-ring-size-converter}}, year = {2026}, note = {TG we-Calculate} }
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