Ring size converter
Pick the system you know, type the size, and see the same ring in every other system. All sizes go through the inner diameter in millimetres, so you can also start from a measured diameter or circumference. Charts differ slightly between jewellers, so use the nearest size as a guide.
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How to use it
- Choose what you have: a US, UK, EU or Japan size, or an inner diameter or circumference in millimetres.
- Type the size. For UK sizes enter a letter such as N or N½.
- Read the diameter, the circumference and the nearest size in each system.
How the conversion works
| System |
Relation to inner diameter |
| US / Canada |
diameter = 11.63 + 0.8128 × size (mm) |
| EU, ISO 8653 |
size = circumference = π × diameter (mm) |
| UK / Australia |
circumference = 37.8 + 1.25 × letters after A (mm) |
| Japan |
diameter = 13 + (size − 1) ÷ 3 (mm) |
Worked example
US size 7 has an inner diameter of 11.63 + 0.8128 × 7 = 17.32 mm. The circumference is π × 17.32 = 54.4 mm, so the nearest EU size is 54, the nearest UK size is N½ and the Japan size is 14.
Tips
- Measure your finger in the evening, when it is largest, and not when it is cold.
- Measuring a ring that already fits is the easiest method: its inner diameter is the number you need.
- The nearest sizes are the sizes jewellers usually stock. In-between sizes often need a resize or a custom order.
Frequently asked questions
How are the sizes converted?
Everything goes through the inner diameter. US diameter = 11.63 + 0.8128 × size mm. EU (ISO 8653) is the inner circumference in mm, so the diameter is size ÷ π. UK letters start at A = 37.8 mm circumference and add 1.25 mm per letter. Japan size 1 is 13 mm and each size adds 1/3 mm.
Why does my jeweller's chart show a slightly different size?
Printed charts round their numbers and are not all based on the same starting values, so a size can differ by up to about half a size. Use the nearest size shown here, and if you are between two sizes, ask the jeweller about the width of the band.
Which measurement should I use, diameter or circumference?
Either. The inner diameter of a ring that fits well and the inner circumference of your finger measured with a strip of paper are linked by circumference = π × diameter (about 3.1416). Enter whichever you measured.
Do wide bands need a bigger size?
Usually yes. A wide band covers more of the finger and fits tighter than a thin one, so many people go up a quarter to half size for bands wider than about 6 mm. The converter does not adjust for width.
What do the half letters in UK sizes mean?
UK sizes go A, A½, B, B½ and so on up to Z. Each half step is 0.625 mm of circumference. Write the half as ½, 1/2 or .5, for example N½, N 1/2 or N.5.