Ring size
Ring size chart: US, UK, EU and millimetres
One table, computed from the standards rather than copied from another chart: the inner diameter and circumference in millimetres for every US size from 3 to 13, with the EU and UK/AUS equivalents. The millimetres are the measure every country agrees on. Below it, three ways to measure a finger at home and the mistakes that put people half a size out.
The chart
Inner diameter is the distance across the inside of the band; circumference is the distance around it, and around the finger. EU sizes are the circumference in millimetres (ISO 8653). Shaded rows are the sizes most rings are stocked in.
| US / Canada | Diameter mm | Circumference mm | EU / ISO | UK / AUS |
|---|---|---|---|---|
| 3 | 14.1 | 44.2 | 44 | F |
| 3.5 | 14.5 | 45.5 | 45 | G |
| 4 | 14.9 | 46.8 | 47 | H |
| 4.5 | 15.3 | 48.0 | 48 | I |
| 5 | 15.7 | 49.3 | 49 | J |
| 5.5 | 16.1 | 50.6 | 51 | K |
| 6 | 16.5 | 51.9 | 52 | L |
| 6.5 | 16.9 | 53.1 | 53 | M |
| 7 | 17.3 | 54.4 | 54 | N½ |
| 7.5 | 17.7 | 55.7 | 56 | O½ |
| 8 | 18.1 | 57.0 | 57 | P½ |
| 8.5 | 18.5 | 58.2 | 58 | Q½ |
| 9 | 18.9 | 59.5 | 60 | R½ |
| 9.5 | 19.4 | 60.8 | 61 | S½ |
| 10 | 19.8 | 62.1 | 62 | T½ |
| 10.5 | 20.2 | 63.4 | 63 | U½ |
| 11 | 20.6 | 64.6 | 65 | V½ |
| 11.5 | 21.0 | 65.9 | 66 | W½ |
| 12 | 21.4 | 67.2 | 67 | X½ |
| 12.5 | 21.8 | 68.5 | 68 | Y½ |
| 13 | 22.2 | 69.7 | 70 | Z½ |
- Half a US size is 0.41 mm of diameter, about 1.3 mm of circumference
- Where a retailer's chart shows a UK letter half a size off this one, it is rounding the same conversion the other way — order by the millimetres
Three ways to measure
With a ring that fits. The most reliable, because it measures the ring rather than the finger. Lay it on a ruler and read inside edge to inside edge in millimetres — the diameter — then find that in the second column. A ring that fits the same finger on the same hand; one from the other hand can be a size out.
With a strip of paper or a length of string. Wrap it around the base of the finger, snug but not tight, and mark where it overlaps. Measure the length to the mark in millimetres: that is the circumference, the third column. Do it three times and take the middle value — string stretches and paper creases, and one reading is a guess.
With a ring sizer. A set of plastic or metal rings in every size, from a jeweller or for a few pounds online. The only method that also tests whether the ring will pass the knuckle, which a strip of paper cannot tell you. The measuring guide goes through each in detail.
What puts people half a size out
Almost every wrong size comes from one of these, and none of them is the ruler.
- Measuring in the morning or in the cold: fingers are smallest then. Measure at the end of the day, warm.
- Ignoring the knuckle: if it is larger than the base, the ring must pass it. Measure both, choose between.
- A wide band: 6 mm and up runs about half a size tight. Comfort-fit bands, rounded inside, fit nearer true.
- The other hand: the dominant hand is usually half a size larger. Measure the hand the ring is for.
- Rounding down: between sizes, go up. Loose can be fixed; too small cannot be worn.
What the systems actually are
US and Canada number the diameter: size 3 is 14.07 mm across and each full size adds 0.8128 mm — a thirty-second of an inch. UK, Australia and Ireland letter the circumference: A is 37.83 mm around and each letter adds 1.25 mm, with half sizes between. EU and most of the world use ISO 8653, which is simply the circumference in millimetres — a 54 is 54 mm around. Japan, India and others have their own numbering; all of them resolve to a circumference, which is why that column is the one to quote across borders.
The three do not line up, and that is the whole reason charts disagree. A US 7 is 54.4 mm around — an EU 54, and between UK N and N½. Round one way and it is N; the other, N½. Neither chart is wrong; the finger is between marks. Order by millimetres and the question does not arise.
For designers: the size is in the file
A ring's size is nothing more than the inner diameter of its band, and in a CAD file that is a number. A STEP or Rhino 3DM carries real units, so a band modelled at 17.3 mm inside is a US 7 in every program that opens it — including the studio here, which renders it at that size. An STL carries no units: it is a list of triangles at whatever scale the exporter assumed, and a ring that was 17.3 mm in Rhino can arrive as 17.3 anything. That is one more reason to keep the 3DM. The guide on ring size in CAD goes through grading a design across sizes. Everything here happens on your own machine: the file is converted and rendered in the browser through your GPU and is never uploaded, so an unreleased design stays unreleased.
Questions
What is my ring size in mm?
Measure the inner diameter of a ring that fits, or the circumference of the finger, and read the row. A US 7 is 17.3 mm across and 54.4 mm around; a US 9 is 18.9 mm across and 59.5 mm around. Millimetres are the one measure every country agrees on, so if you are ordering abroad, give the millimetres.
Why do ring size charts disagree?
Because the systems do not line up. US sizes step the diameter by 0.81 mm; UK letters step the circumference by 1.25 mm; EU sizes are the circumference itself. Converting between them lands between marks, and each chart rounds differently — so two charts can differ by half a UK size for the same finger. This table is computed from the standards rather than copied, and states the millimetres, which are never in dispute.
What is the average ring size?
Most women's rings sell between US 6 and 7 (16.5–17.3 mm across); most men's between US 9 and 10½ (18.9–20.2 mm). These are the common stock sizes, not a rule — hands vary more than the stock does.
Should a wide band be a bigger size?
Yes, usually by half a size. A band 6 mm or wider covers more of the finger and has to pass the knuckle at more points, so it fits tighter than a slim band of the same size. Comfort-fit bands, rounded on the inside, often fit true.
Can a ring be resized?
Most plain and solitaire rings can be resized a size or two either way by a jeweller. Eternity rings set with stones all the way round, tension settings and some tungsten or ceramic rings cannot. Measure carefully before ordering one of those.
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