Ring Size Calculator

Measurement Type doesn't change any downstream formula -- it only decides whether your typed number is treated as a circumference already, or first multiplied by π to convert a diameter into one (line 9); every other calculation on this page works from that single circumference-in-millimeters figure afterward. US Ring Size comes from a linear formula, rounded to the nearest half size, but it's clamped at both ends to the 1.0-16.0 range real US ring sizing charts commonly cover (line 15) -- and unlike a defensive edge-case guard, both clamps are reachable within the calculator's own 10-100mm input range: circumferences above roughly 77mm all report the same 16.0, and circumferences below about 39mm all report the same 1.0, rather than continuing to climb or fall (lines 14-15). UK Ring Size is a letter looked up from a 26-entry alphabet by rounding the circumference into 0.8mm steps starting at 37.8mm (lines 19-22); it does not share the US size's soft half-size rounding -- it always lands on exactly one letter.

Comfort Fit (US) is not recalculated from your raw measurement -- it's the already-clamped US Ring Size plus a flat half size (line 31), so it inherits the same 1.0/16.0 clamp boundaries. Japanese Size uses its own separate linear formula and its own separate 1-30 clamp range (lines 34-35), so it does not saturate at the same circumference the US size does. EU Ring Size is simply the circumference in millimeters, rounded to one decimal -- essentially a direct readout rather than a converted size at all (line 25).