About the Ring Size Converter
Ring sizes are set by the inside circumference or diameter of the band, but each country names them differently: numbers in the US and Canada, letters in the UK and Australia, and millimetres of circumference in the EU and ISO system. This converter takes a size in any of those systems, or a measurement in millimetres, and shows the equivalents in all the others, along with the inside diameter and circumference. It is useful when you are buying a ring from an overseas shop, sizing a gift or checking a chart from a different country.
Updated October 2026
How to use the Ring Size Converter
- 1Choose the system you have the size in.
- 2Type the size, or pick the UK letter.
- 3Read the inside circumference and diameter.
- 4Check the US, UK and ISO sizes below it.
- 5Use the number with the shop's chart before you order.
Measuring tips
Measure in the evening when fingers are warmest, or after a short walk. Avoid measuring when your hands are cold, which makes fingers smaller and leads to a ring that is too tight later. The ring should slide over your knuckle with a little resistance and come off with a gentle twist.
If you are between sizes, choose the larger one, since a ring that is slightly loose can be fitted with a guard but a tight ring is uncomfortable.
For a surprise gift, borrow a ring the person already wears on the same finger and measure its inside diameter with a ruler, or press it into a bar of soap or a piece of card to trace the inside. Enter that diameter in millimetres and the converter returns the circumference and sizes in all the main systems. Jewellers can resize many rings by one to two sizes, but eternity bands and some engraved rings can't be resized, so accurate measuring matters more for those.
Size equivalents
- US 5: 15.7 mm diameter, 49.3 mm circumference, UK J.
- US 6: 16.5 mm, 51.9 mm, UK L½.
- US 7: 17.3 mm, 54.4 mm, UK N½.
- US 8: 18.1 mm, 57.0 mm, UK P½.
- US 9: 18.9 mm, 59.5 mm, UK R½.
Related tools
The Shoe Size Converter converts footwear sizes, the Unit Converter changes millimetres to inches, and the Circle Calculator finds the circumference from a diameter. The Height Converter converts body measurements.
Features
- US, UK, ISO/EU, diameter and circumference
- Half sizes for UK letters
- Rounded to practical quarter sizes for US
- Checks values against the adult range
- Based on standard sizing formulas
- Free, fast, private
Frequently asked questions
How do I convert a US ring size to UK?
A US size 7 is about a UK N½, and a US 5 is about a UK J. The conversion goes through the inside circumference: US size 7 is 54.4 mm. The UK letter whose circumference is closest to that is the match.
How do I find my ring size at home?
Wrap a thin strip of paper or string around the base of the finger, mark where it overlaps and measure the length in millimetres. That is your inside circumference. Alternatively, measure the inside diameter of a ring that fits you.
What is the ISO or EU ring size?
The ISO size is the inside circumference in millimetres, so a ring with a circumference of 54 mm is size 54. Some countries, such as France and Switzerland, subtract 40 from the circumference to give a smaller number.
Why do my fingers need different sizes?
Fingers differ in size, and the dominant hand is usually slightly larger. Temperature, time of day, salty food and exercise also change finger size. Measure when your hands are warm and relaxed, and measure a few times.
Does a wide band need a bigger size?
Often yes. A band 6 mm or wider fits tighter than a thin one because more skin is in contact with it, so many jewellers advise going up a quarter to half a size for wide bands.
Are the conversions exact?
They are close, but sizes are rounded and manufacturers' charts differ slightly. Treat the result as an estimate and compare it with the seller's chart. For an engagement ring, ask a jeweller to measure.