What formulas are used?

US ≈ diameter / 0.794 − 22.7; UK ≈ US − 1.5 (mapped to letters A–Z); EU ≈ 1.2 × diameter + 12.4; ISO = inner circumference in mm.

How are UK letters mapped?

Each whole UK size is one letter (A, B, C, …, Z). Half-sizes also exist but the tool rounds to the nearest letter for clarity.

What is ISO ring size?

ISO 8653 defines ring size as the inner circumference in millimeters. It's the simplest and most universal system.

How should I measure my finger?

Measure at the end of the day when your fingers are warm (slightly larger). Wrap a strip of paper around the finger, mark, then measure length — that's the circumference.

Are the results exact?

They're close approximations. Different jewellers and countries use slightly different tables. For an important purchase, ask the jeweller to confirm.

Is my measurement stored?

No — everything is computed in your browser.

Ring Size Converter

Diameter or circumference → US, UK, EU and ISO ring sizes.

100% private — no uploads Unit & Data ConvertersFree & unlimited Read tutorial

See how it works

Watch a quick simulation of this tool in action — no file needed.

How to use Ring Size Converter

Follow these steps — everything happens instantly in your browser.

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Step 1. Switch to “From diameter” or “From circumference” depending on what you measured.
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Step 2. Enter the value in millimeters.
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Step 3. Read your ring size in US, UK (letter), EU, and ISO formats.