RingSizes

Ring Size in cm — Convert Centimetres to US, UK & EU Sizes

Who this is for: For metric-measure users converting a cm reading, and for anyone who measured a ring's diameter in cm and needs the size it represents.

Type a circumference in centimetres — get the US, UK and EU size with the working shown.

Quick answer: US size = (circumference cm × 10 − 36.5) ÷ 2.55. Examples: 5.0 cm → US 5¼, 5.5 cm → US 7¼, 6.0 cm → US 9¼, 7.0 cm → US 13¼. Measuring a ring instead? Diameter in cm × π = circumference, then convert.

What did you measure?
cm

Paper strip around the finger, measured against a ruler.

US size
7.25
UK
O
EU / ISO
55
Japan / China
14
Circumference55.0 mm = 5.5 cm
Inside diameter17.51 mm = 1.75 cm
French / Italian / GermanFR 15 · IT 15 · DE 17.5
cm charts that step one US size per 0.5 cm are wrong by up to four sizes at the large end — test any chart with its 6.0 cm row: it must read US 9¼.
US size = (circumference mm − 36.5) ÷ 2.55. One 0.1 cm slip moves the answer about 0.39 sizes — measure to the millimetre.
Core facts
Formula(cm × 10 − 36.5) ÷ 2.55
5.0 cmUS 5¼ (EU 50)
5.5 cmUS 7¼ (EU 55)
6.0 cmUS 9¼ (EU 60)
Diameter modecm × π = circumference
US 7 in cm5.44 cm circ · 1.73 cm diam

How does the cm ring size calculator work?

It applies the US standard formula to your centimetres: size = (cm × 10 − 36.5) ÷ 2.55. Switch to diameter mode if you measured across the inside of a ring you already own — the calculator multiplies by π to get circumference first, then converts. Every result is shown with its UK letter and EU/ISO millimetre figure so you can cross-check any seller's chart.

Common uses

  • Convert 5.2 cm, 5.8 cm or any cm reading to a US quarter size
  • Measure an existing ring's inside diameter in cm and get its size
  • Check both cm interpretations (circumference vs diameter) before ordering
  • Translate between your cm measurement and a JP or FR listed size

Frequently Asked Questions

Is ring size measured in cm or mm?
Professionally, millimetres — quarter-size precision needs them. cm works if you keep two decimals: 5.46 cm and 5.5 cm straddle a quarter size.
What ring size is 5 cm in circumference?
5 cm = 50 mm → (50 − 36.5) ÷ 2.55 = US 5¼ (UK K, EU 50).
How many cm is a US 7 ring?
5.44 cm of inside circumference (54.4 mm), or 1.73 cm inside diameter. A US 7 finger measures about 5.4 cm around.
My cm measurement gives a half size — round which way?
Up. Between two sizes, the larger one is almost always the better call: rings are easier to size down than up, and fingers swell through the day.

More ring size calculators

Ring Size Calculator
Circumference, diameter or an existing ring in — get your US, UK, EU, French and Japanese size instantly.

Related guides

  • How to Measure Ring SizeFour accurate home methods — existing ring, paper strip, calibrated printable, and the gift secret-weapon.
  • The Complete Ring Size GuideEvery sizing system explained, quarter sizes, width and knuckle rules, and what resizing costs.
  • 7 cm in Ring SizeExactly what 7 cm of circumference converts to — with the math many blogs get wrong.
  • Average Ring SizeWhat the average man and woman wear, how build shifts it, and how to guess well for gifts.
  • Ring Size 8Everything about US size 8: exact millimetres, international equivalents, and who wears it.