Ring Size Chart & Converter (US, UK, EU)
Enter a ring's internal diameter or circumference in millimeters -- or start from a US size -- to get matching US/Canada, UK/Ireland/Australia/New Zealand and EU/ISO ring sizes, plus a full reference table and two ways to measure at home.
US / Canada size
7.00
UK / Ireland / Australia / NZ
N½
EU / ISO size
54
circumference in mm, rounded
Internal diameter
17.32 mm
Internal circumference
54.41 mm
pi x diameter
Worked example
Say you wrapped a strip of paper around a finger, measured the overlap at 56.5mm, then flattened an existing ring and measured 18.0mm straight across the inside -- that 18.0mm diameter converts to a US size of 7.83, which is closest to UK size P, EU size 57, with an exact circumference of 56.55mm (18.0 x pi). Every figure here comes directly from the calculator above, not typed by hand.
Ring size reference table (US 3–13)
Every row computed from the same table and functions the calculator above uses -- source. The calculator itself supports the full fetched range, US sizes 0 through 16.
| US / Canada | Diameter (mm) | Circumference (mm) | UK / Ireland / AU / NZ | EU / ISO |
|---|---|---|---|---|
| 3 | 14.07 | 44.20 | F | 44 |
| 3.5 | 14.48 | 45.50 | G | 46 |
| 4 | 14.88 | 46.80 | H | 47 |
| 4.5 | 15.29 | 48.00 | I | 48 |
| 5 | 15.70 | 49.30 | J½ | 49 |
| 5.5 | 16.10 | 50.60 | K½ | 51 |
| 6 | 16.51 | 51.90 | L½ | 52 |
| 6.5 | 16.92 | 53.10 | M½ | 53 |
| 7 | 17.32 | 54.40 | N½ | 54 |
| 7.5 | 17.73 | 55.70 | O½ | 56 |
| 8 | 18.14 | 57.00 | P½ | 57 |
| 8.5 | 18.54 | 58.30 | Q¼ | 58 |
| 9 | 18.95 | 59.50 | R½ | 60 |
| 9.5 | 19.35 | 60.80 | S½ | 61 |
| 10 | 19.76 | 62.10 | T½ | 62 |
| 10.5 | 20.17 | 63.40 | U½ | 63 |
| 11 | 20.57 | 64.60 | V½ | 65 |
| 11.5 | 20.98 | 65.90 | W½ | 66 |
| 12 | 21.39 | 67.20 | X½ | 67 |
| 12.5 | 21.79 | 68.50 | Y½ | 69 |
| 13 | 22.20 | 69.70 | Z½ | 70 |
How to measure your ring size at home
String or paper strip method
Wrap a thin strip of paper, or a piece of non-stretchy string, snugly around the base of the finger that will wear the ring -- snug enough to not slide off, but not compressing the skin. Mark the point where the strip first overlaps itself, then lay it flat and measure that length with a ruler in millimeters. That number is your internal circumference -- enter it above in "I know the circumference".
Existing-ring method
Find a ring that already fits the intended finger comfortably (ideally a plain band, not one with a large stone or an open cuff). Lay it flat on a ruler or a millimeter grid and measure straight across the inside of the band, from one inner edge to the opposite inner edge. That number is your internal diameter -- enter it above in "I know the diameter".
Measure a few times and at the end of the day, when fingers tend to be at their largest -- finger size can vary by half a size or more with temperature and time of day.
Where these numbers come from
Source: Wikipedia, "Ring size", "Ring dimensions in various ring size measurement systems" table, sourced to ISO 8653:2016 (fetched 2026-10-01), https://en.wikipedia.org/wiki/Ring_size.
The underlying international standard, ISO 8653:2016, is a paid standard and was not directly fetchable (checked 2026-10-01). Several major jewelers' own size guides were also checked on 2026-10-01 as a possible second source: Brilliant Earth and Tiffany & Co both block automated fetches, James Allen's site now redirects entirely to Blue Nile (a brand consolidation, not a block), and Blue Nile's own guide loads but only as a printable PDF sizer with no extractable numeric table. None offered a usable second source, so this site relies on Wikipedia's freely available, ISO-8653-sourced table instead, with every row's circumference spot-checked against circumference = pi x diameter before use.
Circumference figures this calculator computes live (as opposed to the reference table's own printed values) use circumference = pi x diameter with JavaScript's Math.PI (IEEE-754 double precision, 3.141592653589793).
Frequently Asked Questions
What's the difference between US, UK and EU ring sizes?
They're three different scales measuring the same thing -- a ring's internal circumference. US/Canada sizes are a numeric scale in quarter-size steps (0 to 16+), UK/Ireland/Australia/New Zealand sizes use letters A to Z and beyond (with half sizes), and EU/Continental Europe sizes are simply the internal circumference in millimeters, rounded to a whole number. A US size 7 ring, a UK size N½ ring and an EU size 54 ring are the same physical ring.
How do I measure my ring size at home without a ring sizer?
Two common methods: (1) The string or paper strip method -- wrap a thin strip of paper or string snugly around the base of the finger, mark where it overlaps, then measure that length in millimeters with a ruler. That length is your internal circumference -- enter it into the "I know the circumference" mode above. (2) The existing-ring method -- take a ring that already fits the intended finger, lay it flat, and measure straight across the inside from edge to edge in millimeters. That's your internal diameter -- enter it into the "I know the diameter" mode above. Measure at the end of the day when fingers are warmest and largest, since finger size changes slightly with temperature.
Which existing ring should I measure if I don't know my size?
A ring the intended wearer already wears on the correct finger, ideally one with a plain, even band (not a ring with a large stone or an open-cuff style that could skew the measurement). If no such ring exists, the string/paper method directly on the finger is the fallback.
Why did my measurement land between two listed sizes?
Rings aren't made in every possible fraction of a millimeter -- jewelers typically stock whole and half sizes (and occasionally quarter sizes). If your measured diameter or circumference falls between two standard sizes, this calculator shows the closest listed size; when in doubt between two adjacent sizes, most jewelers recommend sizing up slightly rather than down, since a ring that's a touch loose is easier to adjust than one that's too tight.
Does a ring's band width change what size I should order?
Yes, slightly -- a wider band (think 6-8mm instead of a thin 1-2mm band) touches more of the finger's surface and typically feels tighter than a thin band at the same numeric size, because fingers aren't perfectly cylindrical. Many jewelers suggest going up half a size for a noticeably wide band. This calculator reports the standard ISO-8653-based size; it doesn't adjust for band width, since that adjustment is a jeweler's judgment call, not a fixed formula.
Where do these numbers come from, and how accurate are they?
The underlying international standard, ISO 8653:2016, is a paid standard and was not directly fetchable (checked 2026-10-01). Several major jewelers' own size guides were also checked on 2026-10-01 as a possible second source: Brilliant Earth and Tiffany & Co both block automated fetches, James Allen's site now redirects entirely to Blue Nile (a brand consolidation, not a block), and Blue Nile's own guide loads but only as a printable PDF sizer with no extractable numeric table. None offered a usable second source, so this site relies on Wikipedia's freely available, ISO-8653-sourced table instead, with every row's circumference spot-checked against circumference = pi x diameter before use. Circumference figures this calculator computes live (as opposed to the reference table's own printed values) use circumference = pi x diameter with JavaScript's Math.PI (IEEE-754 double precision, 3.141592653589793). Every row in this site's reference table was individually checked for internal consistency (circumference = pi x diameter) before being used -- see the sourcing note below the reference table for the exact method.
This calculator converts between ring sizing systems using a cited, standards-based reference table (see the sourcing note on this page) and double-precision arithmetic for pi. It is a sizing reference, not a substitute for an in-person measurement by a jeweler -- if you're ordering an expensive or non-resizable ring, confirm the size in person first.