Ring Size Converter

Most size converters treat the German or ISO number as just another scale to translate. It isn't one: ISO 8653:2016 defines the size as the ring's inner circumference in millimetres. A size 54 ring measures 54 mm around the inside, full stop.

That means a German or European shopper does not need this tool to find their own size: it is already printed as a physical measurement on the sizer or the receipt. What they, and everyone else, need help with is translating that measurement into the US numeric scale or the UK letter scale, which is what this tool is for.

The most reliable method uses a strip of paper or string. Wrap it snugly around the base of the finger the ring will sit on, mark the point where it overlaps itself, then lay it flat and measure the marked length with a ruler in millimetres. That figure is the inner circumference.

If you already own a ring that fits well, you can measure its inner diameter instead: lay it flat and measure straight across the inside, then switch this tool to diameter mode.

Measure a warm finger, not a cold one. Fingers are noticeably smaller in the cold or first thing in the morning, and larger later in the day or after exercise, so a size taken from a cold finger will tend to run small once you're warm.

The US scale is a numbered sequence and the UK scale is a letter sequence, A to Z and beyond, both defined as linear functions of the same circumference. This tool applies those exact formulas to whatever measurement you enter, then snaps the US figure to the nearest quarter, the steps the US scale is defined in, showing the exact value alongside it.

The catch is the source charts: different jewellers' UK letter tables disagree by around half a millimetre in where they start, even though they agree on the step size between letters. That is a real disagreement in the industry's own reference material, not a rounding error in this tool. Treat a borderline size as a strong estimate, and confirm it in person, ideally with a jeweller's finger sizer, before you buy.