MM to Ring Size Converter — Finger to US, EU, UK, Japanese Sizes

Formula

How this is calculated

Ring sizing varies by country but all systems trace back to one physical measurement: the inner circumference (or equivalently the inner diameter) of the ring at the finger. To measure at home, wrap a thin strip of paper or a flexible tape measure around the base of the finger you want to size, mark where it overlaps, then measure that length in millimetres — that is your inner circumference. Alternatively, if you have a ring that already fits well, lay it on a ruler and measure the inner diameter across the widest point, then multiply by π to get the circumference.

From the diameter, the US size formula uses an industry-standard linear relationship: US size = (diameter_mm − 11.63) / 0.8128. This was established by the US jewellery trade and gives sizes in half-step increments (e.g. size 6, 6.5, 7). The European/ISO standard (ISO 8653) simply equals the inner circumference in millimetres, rounded to the nearest mm, making it the most direct system. UK/Australian ring sizes use an alphabetical scale starting at A, where each full letter step covers about 1.27 mm of circumference and each half-step (e.g. A½, B½) covers about 0.635 mm. Japanese sizes use a numeric scale that increments by roughly 1.35 mm per size starting at JP 1 ≈ 41 mm.

For the best fit, measure later in the day when fingers are slightly larger, and at room temperature. Fingers can vary by half a size between morning and evening and between hot and cold weather. Wide-band rings (over 6 mm) typically feel tighter and may need to go up half a size.