One measurement. Four ring-size systems.

Use inner circumference as the common reference

International ring-size systems label the same physical ring differently. ISO sizing is based on inner circumference in millimetres, which makes circumference the cleanest bridge between US/Canada, UK/Australia, European and Japanese sizes. Conversions are approximate because regional steps do not always align exactly.

Should you measure the inside or outside?

Measure the inside diameter at the widest point without including the metal. If you use a strip around the finger, measure the snug inner circumference where the ring will sit. A small measurement error can move the result by more than one size, so repeat it.

Why do ring conversion charts disagree?

Systems use different increments, manufacturers work within tolerances and some charts round to the nearest available size. Band profile and width also change fit. A converter is best used to identify the nearest candidate size, followed by confirmation with the specific jeweller.

How to measure for a better fit

Measure at several times of day when hands are at a normal temperature. Fingers can swell in heat, after exercise or during travel and can contract in cold conditions. The ring must pass over the knuckle while sitting comfortably at the base of the finger.

Do wide bands need a larger size?

Often, yes. A wide band contacts more skin and can feel tighter than a narrow ring at the same measured diameter. Very large knuckles, stacking rings and unusually shaped bands also require judgement. Confirm the final fit before a ring is made, engraved or shipped internationally.