Ring Size from Diameter Calculator
Measure a ring you already own and get the US ring size, ISO (circumference) size and matching diameter — from either the inside diameter or the circumference.
How it works
Circumference = diameter × π · US size = (circumference mm − 36.537) ÷ 2.5535 · ISO/EU size = inside circumference in mm
A ring size is really just a measurement of the finger opening. The ISO (and most European) systems quote the inside circumference in millimetres directly, so a 51.9 mm circumference is simply "size 52". The US scale is linear too: each whole size adds about 2.55 mm of circumference (0.81 mm of diameter). Measuring an existing ring and converting is far more reliable than guessing, because finger size drifts with temperature and time of day.
Worked example
A ring with a 16.5 mm inside diameter has a circumference of 16.5 × π = 51.8 mm. Plugging in: (51.8 − 36.537) ÷ 2.5535 ≈ 6.0, so it is a US size 6 — and its ISO/EU size is 52.
Frequently asked questions
Should I measure diameter or circumference?
Either works. Circumference (wrapping a strip of paper around the finger, or measuring around the inside of a ring) is usually more accurate; diameter (measuring straight across the inside of a ring with a caliper) is quick if you already own the ring.
When is the best time to size a finger?
Late in the day at normal room temperature, when fingers are at their average size. Cold shrinks them and heat or salt swells them, which can shift the reading by a full size.
How does this relate to UK ring sizes?
UK sizes use letters rather than numbers, so they do not map cleanly to a formula. Use the inside circumference in millimetres shown here and match it to a UK letter chart for an exact conversion.
Related calculators
This calculator is for educational purposes. Double-check important results before acting on them.