US ring sizes run from about 3 to 13 for adults, and every whole size adds about 0.81 mm of inner diameter (2.55 mm of circumference). A US 7 is 17.3 mm across the inside (0.681 in) and 54.3 mm around. The chart below lists every quarter size, which is how jewellers order wedding and engagement rings in the United States and Canada.
US ring size chart with quarter sizes
| US size | Inner diameter | Diameter (in) | Circumference | Circumference (in) | UK |
|---|---|---|---|---|---|
| 3 | 14.1 mm | 0.555 | 44.3 mm | 1.74 | F |
| 3¼ | 14.3 mm | 0.563 | 44.9 mm | 1.77 | – |
| 3.5 | 14.5 mm | 0.571 | 45.6 mm | 1.79 | G |
| 3¾ | 14.7 mm | 0.579 | 46.2 mm | 1.82 | – |
| 4 | 14.9 mm | 0.587 | 46.8 mm | 1.84 | H |
| 4¼ | 15.1 mm | 0.594 | 47.4 mm | 1.87 | – |
| 4.5 | 15.3 mm | 0.602 | 48.1 mm | 1.89 | I |
| 4¾ | 15.5 mm | 0.610 | 48.7 mm | 1.92 | – |
| 5 | 15.7 mm | 0.618 | 49.3 mm | 1.94 | J½ |
| 5¼ | 15.9 mm | 0.626 | 50.0 mm | 1.97 | – |
| 5.5 | 16.1 mm | 0.634 | 50.6 mm | 1.99 | K½ |
| 5¾ | 16.3 mm | 0.642 | 51.2 mm | 2.02 | – |
| 6 | 16.5 mm | 0.650 | 51.8 mm | 2.04 | L½ |
| 6¼ | 16.7 mm | 0.657 | 52.5 mm | 2.07 | – |
| 6.5 | 16.9 mm | 0.665 | 53.1 mm | 2.09 | M½ |
| 6¾ | 17.1 mm | 0.673 | 53.7 mm | 2.12 | – |
| 7 | 17.3 mm | 0.681 | 54.3 mm | 2.14 | N½ |
| 7¼ | 17.5 mm | 0.689 | 55.0 mm | 2.16 | – |
| 7.5 | 17.7 mm | 0.697 | 55.6 mm | 2.19 | O½ |
| 7¾ | 17.9 mm | 0.705 | 56.2 mm | 2.21 | – |
| 8 | 18.1 mm | 0.713 | 56.9 mm | 2.24 | P½ |
| 8¼ | 18.3 mm | 0.720 | 57.5 mm | 2.26 | – |
| 8.5 | 18.5 mm | 0.728 | 58.1 mm | 2.29 | Q½ |
| 8¾ | 18.8 mm | 0.738 | 58.9 mm | 2.32 | – |
| 9 | 19.0 mm | 0.748 | 59.7 mm | 2.35 | R½ |
| 9¼ | 19.2 mm | 0.756 | 60.3 mm | 2.37 | – |
| 9.5 | 19.4 mm | 0.764 | 60.9 mm | 2.40 | S½ |
| 9¾ | 19.6 mm | 0.772 | 61.6 mm | 2.42 | – |
| 10 | 19.8 mm | 0.780 | 62.2 mm | 2.45 | T½ |
| 10¼ | 20.0 mm | 0.787 | 62.8 mm | 2.47 | – |
| 10.5 | 20.2 mm | 0.795 | 63.5 mm | 2.50 | U½ |
| 10¾ | 20.4 mm | 0.803 | 64.1 mm | 2.52 | – |
| 11 | 20.6 mm | 0.811 | 64.7 mm | 2.55 | V½ |
| 11¼ | 20.8 mm | 0.819 | 65.3 mm | 2.57 | – |
| 11.5 | 21.0 mm | 0.827 | 66.0 mm | 2.60 | W½ |
| 11¾ | 21.2 mm | 0.835 | 66.6 mm | 2.62 | – |
| 12 | 21.4 mm | 0.843 | 67.2 mm | 2.65 | X½ |
| 12¼ | 21.6 mm | 0.850 | 67.9 mm | 2.67 | – |
| 12.5 | 21.8 mm | 0.858 | 68.5 mm | 2.70 | Z |
| 12¾ | 22.0 mm | 0.866 | 69.1 mm | 2.72 | – |
| 13 | 22.2 mm | 0.874 | 69.7 mm | 2.75 | Z+1 |
Quarter sizes are the midpoint between two half sizes. A 6¾ is 17.1 mm, between 6.5 (16.9 mm) and 7 (17.3 mm). Linked sizes have their own page with a true-size circle and more detail.
How US ring sizes are defined
US and Canadian sizes follow a straight line: the inside diameter grows by a fixed step for every size. Jewellers’ charts, ring mandrels and the standard sizing gauges sold in the US all follow the same rule closely:
- Inner diameter (mm) ≈ 11.63 + 0.8128 × US size
- Inner circumference (mm) ≈ 36.5 + 2.55 × US size
- Size 0 would be a ring of about 11.6 mm, and every size after that is roughly 1/32 inch larger in diameter.
Because the scale is linear, a quarter size is always about 0.2 mm of diameter and 0.64 mm of circumference. That is less than the thickness of a credit card, which is why measuring at home needs a little care. The ring size calculator does the maths for you from a diameter or circumference.
US sizes in inches
American ring mandrels and some older charts give the inner diameter in inches. A few anchor points to remember:
| US size | Inner diameter | Inner circumference |
|---|---|---|
| 5 | 0.618 in (about 5/8 in) | 1.94 in |
| 7 | 0.681 in (about 11/16 in) | 2.14 in |
| 9 | 0.748 in (about 3/4 in) | 2.35 in |
| 11 | 0.811 in (about 13/16 in) | 2.55 in |
| 13 | 0.874 in (about 7/8 in) | 2.74 in |
If your tape is in inches, the ring size by diameter page has a lookup that accepts inches.
Which US sizes are most common?
US jewellers report that most women buy between size 5 and 8, with 6 to 7 the busiest range, and most men buy between 8 and 12, with 9 to 10 the busiest. Thumb rings are typically 2 to 3 sizes above a person’s ring-finger size, and pinky rings 1½ to 2 sizes below. See average ring size for the details and how to guess someone’s size for a surprise.
When to choose a quarter size
- You measure between two half sizes and the ring is a wide band (6 mm and over) – order the upper quarter.
- Your knuckle is much larger than the base of your finger. The ring must pass the knuckle, so a quarter up is often the comfortable compromise.
- The ring cannot be resized later. Tungsten, titanium, ceramic and full eternity bands are hard or impossible to resize, so it pays to get the quarter right first time. Read can you resize a ring? before you buy.
Measure your US size
The fastest way is the online ring sizer: calibrate with a bank card, place a ring that fits on the screen and read the US size (plus UK, EU, India and mm). Prefer paper? The US Letter version of the printable ring sizer has circles from US 4 to 12.5.
Not sure of your ring size? Measure it on your phone screen in 30 seconds.
Open the free ring sizer →Related
- UK ring size chart (letters)
- Men’s ring size chart and women’s ring size chart
- Ring size converter
- Ring size by circumference
Frequently asked questions
How many mm is one US ring size?
One whole US size adds about 0.81 mm of inner diameter and about 2.55 mm of inner circumference. A half size is 0.4 mm of diameter.
What is a size 7 ring in inches?
US 7 has an inner diameter of 0.681 inches (17.3 mm) and an inner circumference of 2.14 inches (54.3 mm).
Do quarter ring sizes exist?
Yes. Most US jewellers can order wedding bands and engagement rings in quarter sizes such as 6¼ or 6¾. Fashion rings are usually made only in whole and half sizes.
What is the difference between US size 6.5 and 7?
US 6.5 is 16.9 mm inner diameter and US 7 is 17.3 mm, a difference of 0.4 mm across or about 1.3 mm around the finger.
Are US and Canadian ring sizes the same?
Yes. Canada and Mexico use the same numeric scale as the United States.