RingSizes

Ring Size Calculator — Circumference, Diameter & Reverse Lookup

Who this is for: For anyone buying a ring online, sizing a surprise gift, or checking an heirloom against a modern chart — no sizer kit required.

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

Quick answer: Ring size = (finger circumference in mm − 36.5) ÷ 2.55. A 54.4 mm finger is a US 7 (UK N½, EU 54); 17.3 mm inside diameter is also US 7. Measure at day's end, round up between sizes, and add a quarter size for bands 6 mm wide or wider.

I have…
mm

Measure at day's end — fingers are largest then.

mm

Only if knuckles measure a full size larger than the finger base.

US size
7
UK
N½
BS 6820
EU / ISO
54
circumference mm
Japan / China
13
East-Asia
Circumference54.4 mm · 5.44 cm · 2.14 in
Inside diameter170.90 mm · 17.09 cm
French / ItalianFR 14.3 · IT 14
GermanDE 17.5
Wide bandA 2mm band is narrow enough to keep size 7 as measured.
Between sizes take the larger — rings are easier to size down. Wide bands (≥6 mm) grip more contact area: the +0.25 is already applied above when you pick a wide band.
US formula: size = (circumference mm − 36.5) ÷ 2.55 · diameter: size = (mm − 11.63) ÷ 0.8128. Between sizes, take the larger.
Core facts
US formulasize = (circ mm − 36.5) ÷ 2.55
US 7 anchor54.4 mm circ · 17.3 mm diam
Diameter formulasize = (diam mm − 11.63) ÷ 0.8128
Wide bands≥6 mm wide → +0.25 size
Best timeEnd of day, fingers at largest
Data basisISO 8653 + US standard

What does this ring size calculator do?

It converts one measurement into every ring size system that matters. Measure the circumference of your finger (string or paper strip), the inside diameter of a ring that already fits, or type a size you've been given — the calculator returns US, UK, EU/ISO, French, German and Japanese sizes with the millimetre math shown. The US formula is circumference = 2.55 × size + 36.5, the same relation jewellers' charts are printed from.

Common uses

  • Convert a string measurement (in mm or cm) into your US ring size
  • Check the inside diameter of a ring that fits and find matching sizes
  • Reverse-lookup: start from a US size and read UK, EU, French and Japanese equivalents
  • Compare a seller's chart (often EU or Japanese) against your known US size

Why one formula beats memorising charts

Every printed ring chart is a snapshot of the same line: circumference = 2.55 × US size + 36.5. Charts differ only in how they round. When you compute from your own millimetres you skip the rounding stack — the chart maker rounded, the printer rounded, and you eyeballed between lines. Enter your measurement here and you get the same answer a jeweller's stick sizer would give, plus every international equivalent computed from the same source number.

Frequently Asked Questions

How do I measure my finger circumference for this calculator?
Wrap a strip of paper or non-stretch string around the base of your finger, mark where it overlaps, and measure the length in millimetres. Measure at the end of the day when fingers are largest, and measure 2–3 times — take the average.
What is the most accurate ring size method?
Measuring the inside diameter of a ring you already wear on the same finger. It removes the string-tension error entirely: lay the ring on a ruler, read the inner diameter in mm, and the calculator does the rest.
My measurement falls between two sizes — which do I pick?
Round up to the larger quarter size. A slightly loose ring can be resized down cheaply; a tight one needs metal added. For wide bands (6 mm and up), add a quarter size on top of the rounded value.
Does ring size differ between fingers?
Yes — the ring finger of your dominant hand is typically a quarter to half size larger than the same finger on the other hand. Always size the exact finger the ring will live on.

More ring size calculators

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

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.