Ring Size Converter

How It Works

US ring sizes are defined by the inside diameter of the band: diameter in millimetres = 11.63 + 0.8128 × US size. Each whole size adds about 0.81 mm of diameter, and the circumference is simply the diameter multiplied by π. So the three numbers always travel together: a US size 6 is 16.5 mm across the inside and 51.9 mm around it. When you enter a measurement instead of a size, the converter works the formula backwards and rounds to the nearest quarter size, the finest step jewellers stock.

Worked Example

A US size 6 ring has an inside diameter of 11.63 + 0.8128 × 6 = 16.5 mm, and a circumference of 16.5 × π = 51.9 mm. One size up, a US 7 is 11.63 + 0.8128 × 7 = 17.3 mm across and 54.4 mm around. Going the other way, a ring that measures 17.3 mm across the inside gives (17.3 − 11.63) ÷ 0.8128 = 6.98, which rounds to US size 7.

How to Use This Converter

Choose what you know — a US size, an inside diameter, or a circumference in millimetres — and enter the value. The result shows all three figures at once. To measure a ring you already own, measure across the inside of the band in millimetres. To measure a finger, wrap a strip of paper around the base of the finger, mark where it overlaps, and measure that length — that is the circumference.

Tips and Practical Advice

Measure at the end of the day, when your fingers are at their largest, and measure twice — fingers change size with temperature and time of day, and a ring that fits in the morning can feel tight by evening. Wide bands fit more snugly than thin ones of the same size, so consider going up a quarter size for a band wider than about 6 mm. Charts from different makers can disagree by a fraction of a millimetre, which is normal: for an expensive purchase, a jeweller's ring sizer is worth more than any chart.

Standard US Ring Size Chart

Source: standard US ring-size definition (diameter 11.63 mm + 0.8128 mm per size; circumference = diameter × π), checked against published jewellers' charts 13 November 2026. Values rounded to 0.1 mm; charts may differ by a tenth of a millimetre.