Ring Size Calculators — Convert Any Measurement Instantly

Calculators answer 'what is MY size?' from a measurement you already have: the main calculator takes circumference, diameter or an existing ring and runs it four ways; the cm calculator speaks centimetres natively for metric tapes. Both apply the standard US formula live in your browser — size = (circumference mm − 36.5) ÷ 2.55, or size = (diameter mm − 11.63) ÷ 0.8128 — and return US, UK, EU/ISO, French, German and Japanese equivalents with the working shown. The main calculator also handles the two corrections that move real-world fits: wide bands (6 mm and up take a quarter size more) and large knuckles (size to the knuckle minus a quarter when it measures a full size over the finger base). Measure at the end of the day when fingers are largest, repeat two or three times, and when a result lands between sizes, take the larger — rings are easier to size down than up.