Ring Size Calculator – Find or Convert Your Ring Size
Find your US ring size from finger diameter or circumference.
About the Ring Size Calculator
What is the Ring Size Calculator?
A ring size calculator converts a finger's diameter or circumference into a US ring size.
Why use this calculator?
Buying a ring as a surprise needs a size in advance, and measuring an existing ring or a finger gives a good starting point.
How to use it
- Choose whether you measured the diameter or the circumference.
- Enter the measurement in millimetres.
- Read the equivalent US size.
Formula
Diameter and circumference are linked by π; the US size follows a linear scale of about 0.81 mm per size.
Fingers change size with temperature and time of day, so measure at the end of the day for the largest size.
Frequently Asked Questions
How do I measure an existing ring?
Measure the inside diameter in millimetres and enter it as the diameter.
Do sizes vary by country?
Yes. This gives US sizing; many countries use the ISO/European system, shown here as an approximation.
Can I print or share my result?
Yes. Use your browser's print function or screenshot the result. The page is designed to print cleanly.
What if my inputs are outside the typical range?
The calculators accept a wide range of values, but extreme inputs may produce unrealistic results. Double-check the units you are using.
How often is the calculator updated?
We review formulas and rates periodically to keep them aligned with current standards and published data.
Is this calculator free to use?
Yes. Every calculator on this site is completely free, with no hidden fees, no subscriptions and no limits on how many times you can use it.
How accurate are the results?
We use standard, widely accepted formulas and calculate in your browser using double-precision arithmetic. Results are rounded for readability, so treat them as reliable estimates rather than a guarantee. Real-world factors can vary the true figure.
Can I add this calculator to my own website?
Yes. Open this calculator page and use the "Add to your website" box in the sidebar to copy a ready-made iframe snippet, then paste it into any HTML page.
Does it work on a phone or tablet?
Yes. The layout is fully responsive, so it works on desktops, laptops, tablets and mobile phones, and every input adapts to touch screens.
Is the information I type kept private?
Yes. All calculations run locally in your browser. Nothing you enter is uploaded, stored or shared with anyone.