💍 Ring Size Converter
By ToolNimba Editorial Team · Updated 2026-06-19
Pick what you know on the left, then a size or measurement on the right. The matching size in every other system appears here.
Ring sizing varies slightly between jewellers and chart standards. Treat these as close equivalents and confirm with a jeweller before buying.
This ring size converter matches a ring size in one system to its closest equivalent in every other system. Choose what you already know, a US, UK or EU size, or a measured inner circumference or diameter in millimetres, and the tool reads off the nearest standard size in all the others. It is built from a fixed lookup table rather than a single formula, so the conversions line up with the charts jewellers actually use.
What is the Ring Size Converter?
Ring sizes describe the inside of the band, not the outside, so the only thing that really matters is the inner circumference of the ring (or the diameter, which is the circumference divided by pi). Every regional system is just a different label for the same physical measurement. The US and Canada use a numeric scale where each whole size is about 0.8 mm of circumference apart, and half sizes sit in between. The UK and Ireland use a letter scale (A, B, C and so on, often with a half size shown as a fraction). Continental Europe commonly uses the ISO scale, where the number is essentially the inner circumference in millimetres.
Because all three systems map back to the same circumference, converting between them is a lookup rather than a calculation: find the row whose circumference matches your known size, then read across to the column you want. That is exactly what this tool does. The reason a lookup table beats a single formula is that the regional scales do not increment in perfectly even steps, and rounding conventions differ, so a table built from published charts is more faithful than forcing one equation onto every system.
The catch is that there is no single global standard. Different jewellers, mints and chart publishers round slightly differently, and a half size here can equal a quarter size there. A converted size will almost always be within one size of correct, which is close enough to start from, but it is not a substitute for being measured. Finger size also changes with temperature, time of day and which hand, so for an expensive ring it is worth being sized in person, ideally late in the day when fingers are at their largest.
When to use it
- Buying a ring from an overseas retailer that lists sizes in a different system than your own.
- Converting a UK letter size to the US number you need for an online order, or the other way around.
- Turning a measured inner circumference or diameter in millimetres into a named ring size.
- Checking the EU (ISO) size that matches a ring you already own before ordering a matching band.
How to use the Ring Size Converter
- Choose what you already know in the first menu: US, UK or EU size, or inner circumference / diameter in mm.
- Pick the size from the list, or type your measured millimetre value.
- Read the matching size shown across US, UK, EU, diameter and circumference.
- If your measurement falls between sizes, the tool shows the nearest standard size and flags it.
Formula & method
Worked examples
You wear a US size 7 ring and want the UK and EU equivalents.
- Find the row for US 7 in the ring size chart.
- That row has an inner diameter of 17.3 mm and circumference of 54.4 mm.
- Read across the same row to the UK and EU columns.
- UK shows N 1/2 and EU (ISO) shows 54.
Result: US 7 = UK N 1/2 = EU 54 = 17.3 mm diameter (54.4 mm circumference)
You wrapped paper around your finger and measured a 60.8 mm circumference.
- Divide by pi to check the diameter: 60.8 ÷ 3.1416 = 19.4 mm.
- Look for the chart row closest to 60.8 mm circumference.
- The nearest standard row is 60.8 mm (19.4 mm diameter).
- Read across to the size columns for that row.
Result: 60.8 mm circumference ≈ US 9.5 / UK S 1/2 / EU 61
Ring size conversion chart: US, UK, EU, inner diameter and circumference
| US | UK | EU (ISO) | Inner diameter (mm) | Inner circumference (mm) |
|---|---|---|---|---|
| 4 | H | 46 | 14.8 | 46.5 |
| 5 | J 1/2 | 49 | 15.7 | 49.3 |
| 6 | L 1/2 | 52 | 16.5 | 51.9 |
| 7 | N 1/2 | 54 | 17.3 | 54.4 |
| 8 | P 1/2 | 57 | 18.1 | 57.0 |
| 9 | R 1/2 | 59 | 19.0 | 59.5 |
| 10 | T 1/2 | 62 | 19.8 | 62.1 |
| 11 | V 1/2 | 65 | 20.6 | 64.6 |
| 12 | X 1/2 | 67 | 21.4 | 67.2 |
Common mistakes to avoid
- Measuring the outside of the band instead of the inside. Ring size is based on the inner diameter and circumference. Measuring the outer edge adds the band thickness on both sides and gives a size that is too large. Always measure the inside of an existing ring.
- Treating one chart as a universal standard. Jewellers, mints and websites round sizes slightly differently, so a converted size can be off by up to a size. Use the result as a starting point and confirm with the retailer you are buying from.
- Sizing a cold finger first thing in the morning. Fingers shrink in the cold and swell later in the day. Measuring when your hands are cold can give a size that ends up too tight. Size late in the day at a normal room temperature.
- Ignoring band width. A wide band feels tighter than a thin one at the same nominal size. For bands wider than about 6 mm, many people size up by a quarter to a half size for comfort.
Glossary
- Inner diameter
- The distance straight across the inside of the ring, measured in millimetres. Diameter equals circumference divided by pi.
- Inner circumference
- The distance all the way around the inside of the ring, the measurement that most directly defines size.
- US size
- The numeric ring scale used in the US and Canada, where whole sizes are about 0.8 mm of circumference apart, with half sizes between.
- UK size
- The British and Irish ring scale that uses letters (A, B, C and so on), often with a half size shown as a fraction.
- EU (ISO) size
- The Continental European scale where the size number is essentially the inner circumference of the ring in millimetres.
Frequently asked questions
How do I convert a US ring size to a UK ring size?
Pick US in the first menu and choose your size, and the tool shows the matching UK letter instantly. As a guide, US 7 is about UK N 1/2 and US 9 is about UK R 1/2. Because charts round differently, treat the result as the closest equivalent and confirm with the seller.
How do I measure my ring size at home?
Wrap a thin strip of paper or string snugly around the base of your finger, mark where it overlaps, and measure that length in millimetres. That is your inner circumference. Enter it here and the tool reads off the nearest US, UK and EU size.
Is EU ring size the same as circumference in millimetres?
Close, yes. The EU (ISO) scale is defined as the inner circumference of the ring in millimetres, so an EU 54 has roughly a 54 mm inner circumference. Small rounding differences between charts mean it may be off by a fraction of a millimetre.
My measurement falls between two sizes, which should I choose?
When a measurement sits between standard sizes the tool shows the nearest one and flags it. If you are between sizes it is usually safer to size up slightly, especially for a wider band, so the ring slides over the knuckle comfortably.
Why do ring size charts disagree with each other?
There is no single global standard. Different jewellers, mints and websites use slightly different rounding and step sizes, so the same finger can map to marginally different labels. This converter uses common published values, but always cross-check with the retailer.
Does finger size change during the day?
Yes. Fingers swell in heat and after activity and shrink in the cold, so size can vary by up to half a size through the day. For an accurate fit, measure late in the day when your hands are warm and at their largest.