International Ring Size Converter
Convert ring sizes between US/Canada, UK, EU/ISO, Japan, and diameter or circumference in mm.
Includes how to measure your ring size at home.
International Ring Sizing Systems
Ring sizes around the world are measured using completely different systems — all based on the inner circumference or diameter of the ring.
Diameter and Circumference The fundamental measurement is the inner diameter of the ring (in mm). Circumference = π × diameter (using π ≈ 3.14159).
US / Canada Sizing US sizes run from 3 to 13.5, with half-size increments. Each full US size equals approximately 0.8 mm of additional diameter. US size 7 (the most common women’s size) corresponds to an inner diameter of about 17.3 mm.
UK / Australia / South Africa Sizing Letters are used (A through Z6). Each letter corresponds to 1.25 mm of circumference (approximately). UK size N corresponds to a 17.3 mm diameter, making it equivalent to US size 7.
EU / ISO Sizing The EU (and ISO 8653) system uses the inner circumference in millimeters directly, making it the most precise and unambiguous system. US size 7 ≈ EU size 54 (54 mm circumference).
Japanese Sizing Japan uses numbers 1 through 30, where each number represents an increase of about 0.8 mm in circumference. JP size 14 ≈ US size 7.
How to Measure at Home The easiest method uses a strip of paper: wrap it around the base of your finger, mark where it overlaps, then measure the length with a ruler. That length is your finger circumference in mm. For a ring, measure the inside of the ring band instead.