Ring Size Converter
Find your ring size from the inside diameter or circumference of a ring that fits, or convert between US, UK letter and EU sizes.
US / Canada
7
- UK / Australia / India
- N½
- EU / ISO
- 54.5
- Inside diameter
- 17.3 mm
- Inside circumference
- 54.3 mm
A ring with a 17.3 mm inside diameter is US 7, UK N½ and EU 54.5.
How it's worked out
- Diameter
as measured17.3 mm - US size
(17.3 − 11.63) ÷ 0.81286.98 - UK letter
(54.35 − 37.5) ÷ 1.25 steps from AN½
Ring size chart
| US | Diameter (mm) | Circumference (mm) | UK |
|---|---|---|---|
| 4 | 14.9 | 46.8 | H½ |
| 4.5 | 15.3 | 48 | I½ |
| 5 | 15.7 | 49.3 | J½ |
| 5.5 | 16.1 | 50.6 | K½ |
| 6 | 16.5 | 51.9 | L½ |
| 6.5 | 16.9 | 53.1 | M½ |
| 7 | 17.3 | 54.4 | N½ |
| 7.5 | 17.7 | 55.7 | O½ |
| 8 | 18.1 | 57 | P½ |
| 8.5 | 18.5 | 58.2 | Q½ |
| 9 | 18.9 | 59.5 | R½ |
| 9.5 | 19.4 | 60.8 | S½ |
| 10 | 19.8 | 62.1 | T½ |
| 10.5 | 20.2 | 63.3 | U½ |
| 11 | 20.6 | 64.6 | V½ |
| 11.5 | 21 | 65.9 | W½ |
| 12 | 21.4 | 67.2 | X½ |
- Measure a ring that already fits that finger, across the inside. Fingers swell in the heat and late in the day, and wide bands need about half a size bigger.
The formulas
US size = (diameter − 11.63) ÷ 0.8128 · UK letter = (circumference − 37.5) ÷ 1.25 steps from A · EU = circumference in mm
Example
A ring 17.3 mm across the inside has a circumference of 54.3 mm: US 7, UK N½, EU 54.5. US 5 is about J½.
Fingers are bigger when warm and later in the day, so measure at a normal temperature in the afternoon.
Questions people ask
How do I measure my ring size at home?
Measure across the inside of a ring that fits that finger, in mm. Or wrap a strip of paper round the finger, mark where it meets, and measure the length for the circumference.
What US ring size is 17.3 mm?
US 7, which is N½ in UK sizes and 54.5 in EU sizes.
How do UK ring sizes work?
They are letters from A to Z with half sizes, each step adding 1.25 mm to the circumference. Australia, India and Pakistan jewellers often use the same letters.
Should a wide band be a bigger size?
Yes, usually about half a size bigger, because a wide band fits more tightly.
About this calculator
Formula and rules last checked 25 October 2026Written and tested by the CalculatorDex teamReport a problem