Ring Size Converter
Convert between US, UK and EU ring sizes, or enter your finger circumference or a ring's inner diameter in millimetres to find your size in all systems at once.
Ring Size Converter (US / UK / EU / mm)
Ring Size Formula
Where:
- \(C\) — inner circumference of the ring (mm)
- \(d\) — inner diameter of the ring (mm)
- \(\mathrm{US}\) — numeric US / Canada ring size
- \(n_{\mathrm{UK}}\) — UK half-letter index (A = 0, A½ = 1, B = 2, …)
- \(\mathrm{EU}\) — EU / ISO size, which equals the inner circumference in whole millimetres
Every conversion goes through the ring's inner circumference. Whatever system you start from, the calculator first finds \(C\), then converts \(C\) into all the other systems. The UK letter scale starts at A = 37.8 mm and steps by 0.638 mm per half letter.
Ring Size – Calculation Example
Example: you know your US ring size is 7 and want the UK letter, the EU size and the metric dimensions.
\[ d = 11.63 + 0.813 \times 7 = 17.32 \ \mathrm{mm} \]
\[ C = \pi \times 17.32 = 54.4 \ \mathrm{mm} \]
\[ \mathrm{EU} = 54, \qquad n_{\mathrm{UK}} = \frac{54.4 - 37.8}{0.638} \approx 26 \ \Rightarrow\ \text{letter N} \]
Result: US size 7 = UK size N = EU size 54, with an inner circumference of 54.4 mm and an inner diameter of 17.32 mm.
How to Use the Ring Size Converter
- Choose the From system that matches the size you already know: US / Canada (numeric), UK / Australia (letter), EU / ISO (numeric), finger circumference in mm, or ring inner diameter in mm.
- If you selected the UK system, pick your UK letter size from the drop-down list; otherwise the numeric field stays active.
- Enter the value you know in the Ring size field (for example, 7 for a US size, 54 for an EU size, or 54.4 for a circumference in millimetres).
- Read your equivalents in the results: US / Canada size, UK / Australia letter, EU / ISO size, inner circumference in mm and inner diameter in mm, plus practical sizing tips.
What Is a Ring Size and Why Do the Systems Differ?
A ring size is simply a number or letter that describes the inner dimensions of the ring — either the inner circumference or the inner diameter. The physical ring is the same everywhere; only the labeling differs. The US and Canada use a numeric scale in which each full size adds 0.813 mm to the inner diameter. The UK and Australia use letters from A to Z with half steps (A½, B½, …), each half letter adding about 0.638 mm of circumference. The EU / ISO standard is the simplest: the size number is the inner circumference in whole millimetres, so EU 54 means a 54 mm inner circumference.
Because the scales use different step sizes and different reference points, there is no exact one-to-one match between them. A US 7 lands between UK letters N and N½, and an EU 53.6 would be rounded to EU 54. This is why conversions are always approximate to the nearest half size — a small discrepancy that in practice is covered by the tolerance of how a ring actually fits.
The result section also gives the inner circumference and diameter in millimetres. These two numbers are the universal truth behind every system: \( C = \pi d \). If you can measure either one, you can find your size in any country, which is especially useful when shopping from international jewellers or online marketplaces.
Reference Table: Common Ring Sizes
| US / Canada | UK / Australia | EU / ISO | Circumference (mm) | Diameter (mm) |
|---|---|---|---|---|
| 4 | G½ | 47 | 46.8 | 14.88 |
| 5 | I | 49 | 49.3 | 15.70 |
| 6 | K | 52 | 51.9 | 16.51 |
| 7 | N | 54 | 54.4 | 17.32 |
| 8 | P | 57 | 57.0 | 18.13 |
| 9 | R | 60 | 59.5 | 18.95 |
| 10 | T | 62 | 62.1 | 19.76 |
Tips, Assumptions and Limitations
- Measure at the right time. Fingers are smallest in the morning and in cold weather. Measure at the end of the day, when your hands are warm, for the most representative size.
- Measure the correct finger. Dominant-hand fingers are often slightly larger, and the same finger on each hand can differ by half a size or more.
- Between two sizes? Choose the larger one. A ring that is slightly loose is far more comfortable than one that presses into the finger.
- Wide bands need more room. Bands wider than about 6 mm usually feel tighter, so add half a US size (one UK half letter) compared with a narrow band.
- Conversions are approximations. The calculator uses standard, widely accepted formulas, but individual jewellers and brands may size slightly differently. For expensive or engraved rings, have the size confirmed professionally.
- Accepted range. The calculator covers inner circumferences from about 36 to 80 mm, roughly US sizes 0.5 to 17. Values outside this range are rejected as unrealistic.
If you are also sorting out other measurements, our Shoe Size Converter and Clothing Size Converter handle US / UK / EU conversions for apparel and footwear in the same way.
Frequently Asked Questions
How can I measure my ring size at home without a jeweller's sizer?
Cut a thin strip of paper (about 6 mm wide), wrap it snugly around your finger, mark where it overlaps, and measure the length in millimetres — that is your inner circumference, which you can enter directly into the converter. For better accuracy, repeat the measurement two or three times and take the largest value, as a too-tight reading is the most common source of error.
Can I use a ring I already own to find my size?
Yes. Measure the ring's inner diameter in millimetres with a ruler or, ideally, calipers, and enter that value using the 'Ring inner diameter (mm)' option. Make sure you measure straight across the centre, from one inner edge to the opposite inner edge, excluding the band's thickness. A similar ring of the same width also works well as a comparison when trying sizes in a shop.
What is the average ring size for women and men?
For women, the most common sizes are US 6–7 (UK L–N, EU 52–54). For men, US 9–10 (UK R–T, EU 60–62) is typical. These are only averages — healthy adult fingers span roughly US 4 to 13 for women and US 7 to 14 for men, so a larger or smaller size is perfectly normal and no cause for concern.
How do I find someone's ring size without them knowing?
Borrow one of their rings (from the correct finger and hand) and measure its inner diameter, or press it into a bar of soap or trace the inside circle on paper. Alternatively, ask a friend or family member, or casually compare finger sizes. When in doubt, size slightly up — a ring can be professionally made smaller, but enlarging a band is more difficult and sometimes impossible for some designs.
Are ring sizes the same in the US, Canada, and Australia?
No. The US and Canada share the same numeric scale, but Australia, along with the UK, Ireland, and New Zealand, uses the letter scale (A to Z with half steps). So a US 7, a Canadian 7, and an Australian N all describe essentially the same ring, but a US 7 and an Australian 7 are different sizes — always check which system a label refers to before ordering.
Why does my ring feel tighter in summer or after flying?
Fingers change size by up to half a size or more with temperature, hydration, altitude, pregnancy, and weight changes. Heat, salty food, and long flights cause water retention and swelling; cold weather shrinks fingers. If your size fluctuates a lot, aim for a fit that is comfortable at your finger's largest typical state — usually warm afternoons — rather than sizing for a slim morning finger.
How do I convert ring sizes if I only know inches?
If you measured in inches, multiply by 25.4 to get millimetres, then enter the result as a circumference or diameter. For example, a 2.2-inch inner diameter is 2.2 × 25.4 = 55.88 mm, which corresponds to about a US 9. Using millimetres throughout avoids rounding errors, because even a 0.5 mm mistake in measurement can shift your size by a full half size.