Ring Sizes Converter
Convert ring sizes between US/Canada sizes, EU/ISO sizes, Swiss sizes and the inside diameter or circumference in millimeters.
Sizes differ between brands and countries by up to half a size or more, so treat conversions as a starting point and check the brand's own chart or have your finger sized by a jeweler.
18.5 inside diameter (mm) = 58.11946409 EU ring sizes
Most used ring size units
Popular ring size conversions
What is ring size?
A ring size describes the inside of the ring. The US and Canada use numbers from about 3 to 13 in quarter and half sizes, where each full size adds about 0.81 mm to the inside diameter. Most of Europe uses the ISO system, in which the size is the inside circumference in millimeters; Switzerland subtracts 40 from that. The UK, Ireland and Australia use letters (A to Z), and Japan has its own numbers.
To find your size, wrap a strip of paper around the base of your finger, mark where it overlaps, and measure the length in millimeters: that is your EU/ISO size.
All 6 ring size units
Size systems
Measurements
Frequently asked questions
What is US ring size 7 in EU?
About EU/ISO 54.4 (inside circumference 54.4 mm, diameter 17.3 mm).
What is the most common ring size?
For women about US 6–7 (EU 52–54); for men about US 9–10 (EU 59–62).