CM to Ring Size Converter — Finger Circumference to EU, US, UK, JP Sizes
Enter your finger circumference in centimeters to find your ring size in EU/ISO, US, UK letter and Japanese standards — all at once.
cm
Circumference: 52 mm • ⌀ 16.55 mm
- 1
Circumference in mm
5.2 × 10 = 52 - 2
EU / ISO ring size
round(52) = 52EU/ISO size is the inner circumference in mm rounded to the nearest whole number.
How does this calculator work?
Multiply cm by 10 → mm. EU size = mm rounded. US size = (mm − 36.5) ÷ 2.617. UK: nearest letter in the standard chart. JP = (mm − 37.8) ÷ 0.7. Example: 5.2 cm → 52 mm → EU 52, US ≈ 5.9, UK ≈ N, JP ≈ 20.
Formula
How this is calculated
Each major ring size standard is anchored to the inner circumference or diameter of the ring. The EU/ISO standard (used in Germany, France, Spain, Italy and most of continental Europe) uses the inner circumference in millimeters directly as the size number — EU 52 means 52 mm inner circumference, giving a diameter of 52 ÷ π ≈ 16.56 mm. Since 1 cm = 10 mm, the cm-to-mm conversion is the starting point.
US ring sizes follow a separate scale. The formula US = (circumference_mm − 36.508) ÷ 2.617 converts between them; each full US size represents ≈ 2.617 mm in circumference (≈ 0.832 mm in diameter). Half-sizes (e.g., 6.5) are common — round to one decimal place. UK sizes use a letter code (A through Z+6) derived from a published reference table; the nearest letter for your circumference is shown. Japanese sizes use an integer scale: JP ≈ (circumference_mm − 37.8) ÷ 0.7, so JP 1 corresponds to about 38.5 mm circumference.
All formulas assume you have measured the inner circumference of a well-fitting ring or wrapped a paper strip snugly around the finger at its widest point. Finger size changes slightly with temperature and time of day — measure when fingers are at normal body temperature. For wide bands (≥ 6 mm wide), go one size up as wide rings fit more snugly.
Frequently asked questions
Wrap a strip of paper around the base of your finger (or knuckle if wider). Mark where it overlaps and measure the length in cm. Enter that value to get your EU, US, UK and Japanese ring size.
5 cm = 50 mm circumference → EU 50, US ≈ 5.2, UK ≈ K. This is a small ring size, typically fitting slender fingers or pinkies.
Yes. EU/ISO (numeric mm circumference), US/Canada (different numeric scale), UK/Australia (letters A–Z+6), Japan (integers) and others all use distinct systems. This converter shows all four major standards at once.
Also known as
TG we-Calculate Editorial Team. (2026). CM to Ring Size Converter — Finger Circumference to EU, US, UK, JP Sizes [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/cm-to-ring-size-converter
TG we-Calculate Editorial Team. "CM to Ring Size Converter — Finger Circumference to EU, US, UK, JP Sizes." TG we-Calculate. 2026. https://we-calculate.com/calculator/cm-to-ring-size-converter.
TG we-Calculate Editorial Team, "CM to Ring Size Converter — Finger Circumference to EU, US, UK, JP Sizes," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/cm-to-ring-size-converter
@misc{wecalculate_cm_to_ring_size_converter, title = {CM to Ring Size Converter — Finger Circumference to EU, US, UK, JP Sizes}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/cm-to-ring-size-converter}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?