Ring Size Converter
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Convert a ring size between US, UK and Australian, EU/ISO and Japanese systems, or from a measurement in millimetres.
Ring size reference chart
| US / Canada | UK / Australia | EU / ISO | Japan | Diameter (mm) | Circumference (mm) |
|---|
All conversions between systems are approximate. A jeweller’s sizing is the most reliable guide.
How the ring size converter works
Every ring sizing system is really just a way of labelling one measurement: the inside of the ring. Some systems label the inside diameter, some label the inside circumference, and some use letters or numbers that step up by a fixed amount. This converter turns whatever you enter into an inside circumference in millimetres, then finds the closest size in each of the other systems.
Circumference is used as the common base because that is how the international standard, ISO 8653, defines ring sizes. Diameter and circumference are linked by a simple rule: circumference = diameter x pi (about 3.1416).
The conversions it uses
| System | How it is defined |
|---|---|
| EU / ISO | Size number = inside circumference in mm (size 54 is about 54 mm around). |
| US / Canada | Inside diameter = 11.63 mm + 0.8128 mm per whole size. Each whole size adds 0.032 in of diameter, and quarter sizes are common. |
| UK / Australia | Letters from A, with half sizes. Under British Standard BS 6820, each letter adds 1.25 mm of circumference, and size C is 40 mm. |
| Japan | Whole numbers. Size 1 is about 13.0 mm inside diameter, and each size adds about one third of a millimetre (roughly 1 mm of circumference). |
The converter rounds to the nearest size each system normally uses: quarter sizes for US, half sizes for UK and Australia, and whole numbers for EU/ISO and Japan.
Worked example: US size 7
- Diameter: 11.63 + (7 x 0.8128) = 17.32 mm.
- Circumference: 17.32 x 3.1416 = 54.41 mm.
- EU / ISO: 54.41 rounds to 54.
- UK / Australia: (54.41 – 37.5) / 1.25 = 13.5 letters past A, which is N 1/2.
- Japan: (17.32 – 13.0) x 3 + 1 = 13.96, which rounds to 14.
So a US 7 is roughly a UK or Australian N 1/2, an EU 54 and a Japanese 14. That matches common published conversion charts.
How to measure at home
- From a ring that fits: measure straight across the inside of the band, edge to edge, in millimetres. Enter that as inside diameter.
- From your finger: wrap a strip of paper around the base of the finger, mark where it overlaps, and measure the length flat. Enter that as inside circumference.
- Measure at the end of the day, when fingers tend to be slightly larger, and measure more than once.
- Make sure the ring will pass over your knuckle. If the knuckle is much wider than the base of the finger, size for the knuckle.
Limitations
Conversions between systems are approximate. The systems use different step sizes, so a size in one system often falls between two sizes in another. Different jewellers and countries also publish slightly different charts. Some Australian charts, for example, list letter sizes around half a millimetre smaller in circumference than BS 6820 gives. Wide bands usually feel tighter than narrow ones, so you may need to go up a little for a wide band. A paper strip or a ruler is never as accurate as a jeweller’s sizing ring set. For an engagement ring or any ring that is hard to resize, have your finger sized by a jeweller before you buy.
Sources this tool draws on: ISO 8653 (ring sizes defined by inner circumference in millimetres); British Standard BS 6820 (UK letter sizes); the Wikipedia article “Ring size” (US diameter steps and cross-system conversion table); published Japanese ring size charts (Japanese sizes against inside diameter and circumference); Australian jeweller conversion charts, used as a cross-check.
Our pick on Amazon: measure at home
A reusable ring sizer is more reliable than string or paper for the home method above.
- Reusable Ring Sizer Gauge: a plastic sizing tool with US sizes, used like a belt around your finger.
Fingers change size with temperature and time of day. Measure more than once, and confirm with a jeweller for an expensive ring.
Why your ring size can vary
A single measurement is a good start, but fingers are not fixed in size. Understanding what changes them helps you choose a size that is comfortable all day.
Time of day and temperature
Fingers tend to be slightly smaller in the cold and slightly larger in the heat, after exercise, or when you have eaten salty food. For this reason jewellers often suggest measuring at the end of the day, when your hands are at a comfortable, normal temperature, and repeating the measurement on more than one day.
Knuckles and wide bands
If your knuckle is noticeably larger than the base of your finger, a ring has to pass over the knuckle and still stay put at the base. Many people with larger knuckles choose the size that slides over the knuckle with a little resistance. Wide bands also tend to feel tighter than narrow ones, so people often go up by about a quarter to a half size for a band that is much wider than average. A jeweller can advise on the right fit for the style you are considering.
Which rings can be resized
Plain bands in gold, platinum and silver can usually be resized by a jeweller by a size or two. Rings with stones set all the way around the band, known as eternity rings, and some patterned bands are harder or impossible to resize without disturbing the design. Some modern materials, such as tungsten and titanium, generally cannot be resized at all. If you are buying a ring as a surprise, it is worth asking the seller about their resizing and exchange policy before you buy.
Buying online or from another country
Different countries use different sizing systems, which is why the converter above exists. When ordering from an overseas seller, check which system the size is given in, and whether the seller states the inside diameter or circumference in millimetres. Millimetre measurements are the least ambiguous way to confirm a size across systems.
Measuring a ring you already own
If you have a ring that fits the finger in question, you can measure its inside diameter with a ruler in millimetres, or lay a thin strip of paper around the inside and mark where it overlaps to find the circumference. Either figure can then be compared with the chart above. Make sure the ring is a plain band worn on the same finger, as rings worn on different fingers or of different widths may not give the same result.
Frequently asked questions
What is the average ring size?
People often quote ranges, but finger sizes vary widely with hand size, build and which finger you’re measuring. It is better to measure than to guess from an average.
Are UK and Australian ring sizes the same?
Yes, both use the letter system, with half sizes between letters. You might see small differences between charts from different jewellers, so if you’re between two sizes, ask the jeweller which chart they use.
Is EU sizing the same as ISO sizing?
EU sizes on retail charts usually follow the ISO approach, where the size number is the inside circumference in millimetres. Some retailers use their own numbering, so if a size looks unusual, check which system the retailer means.
Why did my converted size come out between two sizes?
Each system steps up by a different amount, so an exact match isn’t always possible. The converter rounds to the nearest available size. If the fit really matters, try on both sizes or ask a jeweller.
Can I print the reference chart?
Yes. When you print this page, the converter form is hidden and the chart prints on its own. Use it as a guide only, not as a sizing tool.