About the Ring Size Converter
This ring size converter finds your size from a simple measurement and translates it between the main sizing systems. Measure the inside diameter of a ring that fits, or wrap a strip of paper around your finger and measure its length (the circumference), then enter it in millimetres. You can also start from a known US size.
The result gives the US and Canadian size (to the nearest quarter), the UK, Irish and Australian letter size, and the EU/ISO size, which is simply the inside circumference in millimetres. It is useful when buying a ring online from another country, resizing an heirloom, or planning a surprise engagement ring from a ring borrowed out of a jewellery box.
Sizes follow the US standard (size 0 = 11.63 mm diameter, +0.8128 mm per size) and the British letter scale (1.25 mm of circumference per letter). Brands and wide bands can fit differently, so treat results between two sizes as a reason to size up.
With the default inputs, the us / canada size is 7. Change any value above to recalculate instantly.
How to use the ring size converter
- 1Wrap a thin strip of paper or string snugly around the base of your finger and mark where it overlaps.
- 2Measure the length in millimetres — that is your circumference. Or measure the inside diameter of a ring that fits.
- 3Choose the matching option and enter the measurement.
- 4Read your US, UK and EU sizes, and check the chart for neighbouring sizes.
Formula and method
A ring’s inside diameter and circumference are linked by C = π × d. US and Canadian sizes rise by 0.8128 mm (0.032 inch) of inside diameter per whole size, starting from 11.63 mm at size 0, so the size is the diameter above 11.63 mm divided by 0.8128, rounded here to the nearest quarter size.
UK, Irish and Australian sizes are letters that step by about 1.25 mm of circumference, with half sizes in between; this converter starts the scale at 37.5 mm for size A. EU and ISO 8653 sizes are simply the inside circumference in millimetres, rounded to a whole number.
- C
- Inside circumference in mm
- d
- Inside diameter in mm
Worked examples
54.4 mm finger circumference
A 54.4 mm circumference is a 17.3 mm diameter, which is US size 7, UK N½ and EU 54.
Ring with an 18.2 mm inside diameter
An 18.2 mm diameter gives a circumference of about 57.2 mm — US 8 (8.08 exactly), UK P½ and EU 57.
Converting US 5.5
US 5.5 has a 16.1 mm inside diameter and a 50.6 mm circumference, which is UK K½ and EU 51.
Frequently asked questions
How do I measure my ring size at home?+
Wrap a strip of paper around the base of your finger, mark where it meets, and measure the length in millimetres. Measure at the end of the day when fingers are largest, and make sure the ring will still slide over your knuckle.
What is the average ring size?+
Commonly quoted averages in the US are about size 6 to 7 for women and 9 to 10 for men, but hands vary widely, so measure rather than guess — especially for an engagement ring.
How do I convert a UK ring size to US?+
Find the UK letter’s circumference and convert. As a rule of thumb, UK L is about US 5¾, UK N½ is US 7, UK P½ is US 8 and UK T is US 9¾. The chart on this page lists every half size.
What is an EU ring size?+
European and ISO ring sizes are the inside circumference in millimetres. EU 54 means a 54 mm circumference, roughly US 7. Some countries such as Switzerland subtract 40 from that number.
Should I size up for a wide band?+
Yes. Bands wider than about 6 mm fit more snugly because they cover more of the finger, so most jewellers recommend going up a quarter to a half size.