Ring Size Converter – US, UK, EU, Japan and mm Chart

How to use Ring Size Converter

1. Choose “Convert a size” or “From a measurement”.
2. Select your size system and size, or enter millimetres.
3. Say whether you measured circumference or diameter.
4. Read the matching size in every system.
5. Check the highlighted row in the chart.

How ring sizes are defined

Every ring size system describes the inside of the band. US and Canadian sizes rise in quarter and half steps, with each full size adding about 0.81 mm to the inner diameter. UK and Australian sizes use letters (with halves), European sizes follow ISO 8653 and equal the inner circumference in millimetres, and Japanese sizes use numbers from 1 to about 30.

The converter calculates diameter from the US size (11.63 mm + 0.8128 mm per size) and circumference as π × diameter.

How to measure your ring size at home

With a ring that fits: measure the inner diameter edge to edge in millimetres and choose “Inner diameter”.

With a strip of paper: wrap it around the base of your finger, mark where it overlaps, lay it flat and measure the length in mm. Choose “Circumference”. Make sure the strip can still slide over the knuckle.

Measure at the end of the day and at room temperature: fingers swell in heat and in the evening and shrink in the cold. Measure the exact finger, as the same finger on each hand can differ by half a size.

Wide bands and engagement rings

Bands wider than about 6 mm feel tighter, so go up a quarter or half size. If you are between two sizes, choose the larger one; a ring that is slightly loose can be adjusted, but one that does not pass the knuckle cannot be worn. For a surprise engagement ring, borrow a ring the person already wears on that finger and measure it.

More size converters

Convert your shoe size or height, and convert millimetres and inches with the Length Converter.

You may also find these free tools useful: Cooking Measurement Converter, Area Converter and Weight Converter.