Ring Circumference to Diameter Converter
Enter a circumference measurement — from a paper strip, string, or existing ring — and instantly get the diameter, radius, and matching ring size.
Diameter, circumference, and radius converter
Enter any one of the three and we'll work out the other two, plus the matching ring size.
The circumference-to-diameter formula
This is a fixed geometric relationship, not a ring-sizing rule.
For any circle, diameter = circumference ÷ π (pi, approximately 3.1416). Whether you wrapped a paper strip around your finger or measured around the inside of a ring, that circumference determines the inner diameter precisely — no guesswork required.
Why you might need this
- You measured your finger's circumference but a chart or supplier asks for diameter.
- You're comparing a paper-strip measurement against a ring's stated inner diameter.
- You just want the underlying geometry, separate from a specific sizing system.
Frequently asked questions
How do I convert ring circumference to diameter?
Diameter is always circumference ÷ π (about 3.1416) — enter your finger or ring circumference above in mm, cm, or inches and the diameter is calculated instantly, along with your matching ring size.
How do I measure my finger’s circumference?
Wrap a thin strip of paper or non-stretchy string snugly around the base of your finger, mark where it overlaps, then measure that length with a ruler — that’s your circumference.
Why does this tool also show a ring size?
Since your circumference already tells us your ring size, we show it as a bonus alongside the pure geometry conversion — no extra input needed.