Reference · United States & Canada
US ring size chart
This page is the US system on its own, in depth: every size from 3 to 13.5 in half steps, the inside diameter and circumference in millimetres, and the linear formula that generates any size in between. If you need to convert a US size into a UK, EU, Indian or Japanese one, theinternational ring size chartputs all five systems in one row instead.
US ring sizes in mm
Half sizes only. Quarter sizes exist and a jeweller can cut them, but they are rarely stocked.
| US size | Ø mm | Circ. mm | UK / AU | EU / ISO |
|---|---|---|---|---|
| 3 | 14.1 | 44.2 | F | 44 |
| 3½ | 14.5 | 45.5 | G | 45 |
| 4 | 14.9 | 46.8 | H | 47 |
| 4½ | 15.3 | 48.0 | I | 48 |
| 5 | 15.7 | 49.3 | J | 49 |
| 5½ | 16.1 | 50.6 | K | 51 |
| 6 | 16.5 | 51.9 | L | 52 |
| 6½ | 16.9 | 53.1 | M | 53 |
| 7 | 17.3 | 54.4 | N | 54 |
| 7½ | 17.7 | 55.7 | O | 56 |
| 8 | 18.1 | 57.0 | P | 57 |
| 8½ | 18.5 | 58.2 | Q | 58 |
| 9 | 18.9 | 59.5 | R | 60 |
| 9½ | 19.4 | 60.8 | S | 61 |
| 10 | 19.8 | 62.1 | T | 62 |
| 10½ | 20.2 | 63.3 | U | 63 |
| 11 | 20.6 | 64.6 | V | 65 |
| 11½ | 21.0 | 65.9 | W | 66 |
| 12 | 21.4 | 67.2 | X | 67 |
| 12½ | 21.8 | 68.5 | Y | 68 |
| 13 | 22.2 | 69.7 | Z | 70 |
Need Indian or Japanese sizes too? Theinternational ring size chartcarries all five systems side by side. Converting to British letters,US to UK ring sizegoes number by number. Below US 3, children’s rings are on thekids ring size chart.
The maths
The US scale is a straight line, so any size can be computed
Circumference from size
circumference (mm) = 36.537 + 2.5535 × US size
A US 7 is 36.537 + 17.87 = 54.4 mm around the inside.
Size from circumference
US size = (circumference − 36.537) ÷ 2.5535
Measured 57 mm with a paper strip? That is a US 8.
Diameter to circumference
circumference = diameter × π
A 17.3 mm inside diameter is 54.4 mm around — a US 7.
This is the ISO 8653 relationship, and it is what thering size calculator uses internally. It is also why one millimetre of circumference error is worth about 0.4 of a US size, and a millimetre of diameter error more than a full size — close enough only if the ruler you measured with was itself accurate.
Questions
US ring sizing, answered
What is the most common US ring size?
US 6 to 6.5 for women and US 10 for men. Retailers stock the band from US 5 to 8 and US 9 to 11 most deeply, which is why sizes outside it often carry a longer lead time.
How many millimetres is each US ring size?
One whole US size is 2.5535 mm of inside circumference, or about 0.81 mm of inside diameter. A half size is therefore roughly 1.28 mm of circumference — small enough that measuring to the nearest half millimetre matters.
Are US and Canadian ring sizes the same?
Yes. Canada uses the same numeric scale as the United States, so a US 7 is a Canadian 7. Mexico also uses a numeric scale, but not the same one — check it against the millimetre column rather than the number.
Do US ring sizes go in quarter sizes?
The scale is continuous and a jeweller can cut quarter sizes, but most ready-made rings are sold in half sizes. If your measurement lands on a quarter, round to the nearest half size, and round up rather than down when it falls exactly between.
What US size is a 17 mm ring?
A 17.0 mm inside diameter is a US 6.5, or 53.4 mm of inside circumference. 18.0 mm is close to a US 8, and 19.0 mm is close to a US 9. The table above gives every step in between.