Ring Size Calculator
Convert a measured finger circumference or diameter into US, UK, and EU ring sizes.
Reviewed by the ToolNestr Editorial Team — July 2026
How ring sizing works
Unlike clothing sizes, which are somewhat arbitrary labels, ring sizes are direct stand-ins for a physical measurement: the inner circumference (or diameter) of the ring. The relationship between the two is simple geometry — circumference equals π (about 3.14159) times diameter — so measuring either one is enough to find your size in any system.
US sizing runs on its own numeric scale from roughly size 3 to 13 for adults, UK sizing uses letters from A to Z, and EU (Continental) sizing uses a number that sits close to the actual circumference in millimeters. Because all three trace back to the same physical circle, converting between them is a lookup against millimeter measurements rather than an approximate offset like clothing sizing.
Size-to-millimeter relationships follow the standard international jewelry sizing chart used across US, UK, and EU (ISO) ring sizing conventions.
Worked example
A string wrapped around a finger measures 54 mm around.
Real-world context
Buying an engagement ring as a surprise is the highest-stakes version of this problem — there's no chance to try it on first, so shoppers borrow an existing ring the person wears on the correct finger, measure its inside diameter with a ruler, and convert. Getting this wrong by even one size means an expensive resize before it can be worn.
International jewelry shopping runs into the same UK-letter-versus-US-number confusion as clothing, but with less room for error, since a ring that's a full size off either won't go over the knuckle or will spin loosely on the finger — there's no "roomy fit" equivalent to a slightly baggy shirt.
Common misconceptions
"All my fingers are the same ring size." Rarely true. Dominant-hand fingers are often slightly larger, and each finger has a different base circumference, so measure the specific finger the ring will be worn on, not a different one.
"Wide bands fit the same as thin bands in the same size." Not exactly. Wider bands touch more of the finger's surface and tend to feel tighter than a thin band of the same labeled size, so many jewelers recommend going up half a size for bands wider than about 6mm.
Related tools
Frequently asked questions
How do I measure my finger for ring size?
Wrap a strip of paper or string snugly around the base of your finger, mark where it overlaps, then measure that length in millimeters with a ruler — that number is your circumference. Measure at the end of the day when fingers are largest, and avoid measuring when cold.
Should I measure circumference or diameter?
Circumference (distance around the finger) is easier and more accurate to measure at home than diameter (distance across), since diameter requires measuring an existing ring rather than your finger directly. This tool accepts either.
Why do knuckles matter for ring sizing?
A ring must pass over the knuckle to reach the base of the finger, so if your knuckle is noticeably larger than the base, size for comfortable passage over the knuckle rather than a snug fit at the base, or consider a ring with a hidden sizing mechanism.
Do ring sizes differ between US and EU the same way as clothing?
No — ring sizing in every major system is ultimately based on the same physical circumference or diameter measurement, just labeled differently (a letter in the UK, a number close to the millimeter circumference in the EU, and a separate numeric scale in the US), so the conversion is a direct physical relationship rather than an approximate offset.