Sizes
Ring size converter
UK letters, circumference, diameter, EU millimetres and an approximate US size.
Ireland uses the same clothing and shoe sizes as the UK. Brands still vary from any chart.
These are standard published conversions. A label from one brand can sit a size either side of the chart.
How to use it
- Choose a UK letter, or type an inner diameter in millimetres.
- Read the circumference, the diameter, the EU millimetre size and the approximate US number.
- Measure the finger later in the day. Knuckles and cold hands both move the result.
Letters, millimetres and the US quarter sizes
UK, Irish and Australian jewellers still sell letters. The step used here is BS 6820:1987: size A is an inner circumference of 37.5 mm, each full letter adds 1.25 mm, and each half letter adds 0.625 mm. L is 51.25 mm and Z is 68.75 mm. Diameter is the circumference divided by π, shown to two decimal places.
The EU and ISO size is the inner circumference in millimetres, rounded to the nearest millimetre. That follows ISO 8653. There is no single official UK-to-US ring table. This page uses diameter in inches, then (inches − 0.458) / 0.032, rounded to the nearest quarter. Treat the US column as that formula. Other charts can sit half a size away.
Measuring a finger or a ring that fits
The reliable check is a ring that already fits. Measure its inner diameter across the centre, in millimetres, and enter that. A paper strip around the finger gives a circumference. Measure at the end of the day, and measure the knuckle as well: the ring has to pass the knuckle and then sit on the finger.
Wide bands fit more tightly than a thin band of the same diameter, so a wide ring often needs a larger letter. Ireland uses the UK letter system. These rows are a published chart. Two brands with the same number can fit differently, and a garment can be cut for a different body than the chart assumes.
Questions
What is a UK L in millimetres?
51.25 mm around the inside. The diameter is that divided by π, about 16.31 mm. The EU size rounds to 51.
Are US ring sizes official conversions of the letters?
The letters and the circumference come from the British Standard step. The US number is a separate formula and is labelled as approximate.