To convert inches to ring size, find your inner diameter or finger circumference in inches in the chart below. For example, an inner diameter of 0.650 inches (16.5 mm) is a US size 6, and an inner circumference of 2.043 inches (51.9 mm) is also a size 6.
Ring Size Chart in Inches
Inner diameter is the width across the inside of the band. Inner circumference is the distance around the inside. Use the diameter column to measure a ring you own and the circumference column to measure your finger.
| US size | Inner diameter (in) | Inner circumference (in) | Inner diameter (mm) | Inner circumference (mm) | UK | EU |
|---|---|---|---|---|---|---|
| 3 | 0.554 | 1.740 | 14.07 | 44.2 | F | 44 |
| 3.5 | 0.570 | 1.791 | 14.47 | 45.5 | G | 45 |
| 4 | 0.586 | 1.843 | 14.88 | 46.8 | H | 47 |
| 4.5 | 0.602 | 1.890 | 15.29 | 48.0 | I | 48 |
| 5 | 0.618 | 1.941 | 15.69 | 49.3 | J½ | 49 |
| 5.5 | 0.634 | 1.992 | 16.10 | 50.6 | K½ | 51 |
| 6 | 0.650 | 2.043 | 16.51 | 51.9 | L½ | 52 |
| 6.5 | 0.666 | 2.091 | 16.91 | 53.1 | M½ | 53 |
| 7 | 0.682 | 2.142 | 17.32 | 54.4 | N½ | 54 |
| 7.5 | 0.698 | 2.193 | 17.73 | 55.7 | O½ | 56 |
| 8 | 0.714 | 2.244 | 18.13 | 57.0 | P½ | 57 |
| 8.5 | 0.730 | 2.291 | 18.54 | 58.2 | Q½ | 58 |
| 9 | 0.746 | 2.343 | 18.95 | 59.5 | R½ | 60 |
| 9.5 | 0.762 | 2.394 | 19.35 | 60.8 | S½ | 61 |
| 10 | 0.778 | 2.445 | 19.76 | 62.1 | T½ | 62 |
| 10.5 | 0.794 | 2.492 | 20.16 | 63.3 | U½ | 63 |
| 11 | 0.810 | 2.543 | 20.57 | 64.6 | V½ | 65 |
| 11.5 | 0.826 | 2.594 | 20.98 | 65.9 | W½ | 66 |
| 12 | 0.842 | 2.646 | 21.38 | 67.2 | X½ | 67 |
| 12.5 | 0.858 | 2.697 | 21.79 | 68.5 | Z | 68 |
| 13 | 0.874 | 2.744 | 22.20 | 69.7 | Z½ | 70 |
UK letters are matched to the nearest published inner diameter. For other conversions, see our ring size conversion chart, US ring size chart and UK ring size chart.
Inches to Ring Size Formulas
You can calculate a US ring size directly. One inch is 25.4 mm, and each full US size adds 0.8128 mm to the inner diameter, starting from a base of 11.63 mm.
- From inner diameter in inches: size = 31.25 x diameter – 14.31
- From finger circumference in inches: size = 9.947 x circumference – 14.31
Worked examples
- A ring with an inner diameter of 0.70 inches: 31.25 x 0.70 – 14.31 = 7.57, so choose US 7.5.
- A finger circumference of 2.25 inches: 9.947 x 2.25 – 14.31 = 8.07, so US 8.
- A finger circumference of 2.125 inches (2 1/8): 9.947 x 2.125 – 14.31 = 6.83, so US 7 or 6.75 if quarter sizes are offered.
Fractional Inches to Ring Size (Ruler Chart)
Most rulers and tapes mark inches in sixteenths. This table converts a finger circumference read from a ruler to a US size. Round to the nearest half size, then check the fit.
| Finger circumference | Millimetres | Exact US size | Nearest half size |
|---|---|---|---|
| 1 3/4 in | 44.4 | 3.10 | 3 |
| 1 13/16 in | 46.0 | 3.72 | 3.5 |
| 1 7/8 in | 47.6 | 4.34 | 4.5 |
| 1 15/16 in | 49.2 | 4.96 | 5 |
| 2 in | 50.8 | 5.59 | 5.5 |
| 2 1/16 in | 52.4 | 6.21 | 6 |
| 2 1/8 in | 54.0 | 6.83 | 7 |
| 2 3/16 in | 55.6 | 7.45 | 7.5 |
| 2 1/4 in | 57.1 | 8.07 | 8 |
| 2 5/16 in | 58.7 | 8.69 | 8.5 |
| 2 3/8 in | 60.3 | 9.32 | 9.5 |
| 2 7/16 in | 61.9 | 9.94 | 10 |
| 2 1/2 in | 63.5 | 10.56 | 10.5 |
| 2 9/16 in | 65.1 | 11.18 | 11 |
| 2 5/8 in | 66.7 | 11.80 | 12 |
| 2 11/16 in | 68.3 | 12.43 | 12.5 |
| 2 3/4 in | 69.8 | 13.05 | 13 |
One sixteenth of an inch is 1.59 mm, which equals about 0.6 of a ring size. A reading that is off by one mark can change your size by more than a half size, so measure carefully and repeat.
How to Measure in Inches
Measure finger circumference
- Cut a strip of paper about 1/2 inch wide and 6 inches long.
- Wrap it around the base of your finger and mark where it overlaps.
- Lay the strip flat and measure from the end to the mark in inches.
- Find the value in the circumference column or use the formula.
Measure an existing ring
- Choose a ring that fits the same finger well.
- Measure the inside width of the band, not the outside.
- Find the value in the inner diameter column.
For the full process, see how to measure your ring size and print our printable ring size chart.
Convert Millimetres and Centimetres to Inches
Divide millimetres by 25.4 to get inches. For example, 16.5 mm is 0.650 inches. For centimetres, see what is a 6.5 cm ring size and for a worked inch-to-mm example see 2 5/8 inches to mm.
Common Mistakes When Converting Inches to Ring Size
- Mixing up diameter and circumference. A size 6 is 0.650 inches across but 2.043 inches around. Circumference is about 3.14 times the diameter.
- Measuring the outside of the ring. Band thickness adds to the outer width, so measure the inside.
- Rounding fractions too early. Convert to millimetres first, then round.
- Ignoring fit. A wide band or large knuckle may call for a half size up.
Inches to Ring Size FAQs
How do I convert inches to ring size?
Find your inner diameter or finger circumference in inches in the chart and read the US size across. For a formula, size = 31.25 x diameter in inches minus 14.31, or size = 9.947 x circumference in inches minus 14.31.
What is a size 7 ring in inches?
A US size 7 ring has an inner diameter of 0.682 inches (17.32 mm) and an inner circumference of 2.142 inches (54.4 mm).
How do I convert 16.5 mm to inches for ring size?
Divide 16.5 by 25.4 to get 0.650 inches. A 16.5 mm inner diameter is a US size 6.
Is ring size based on diameter or circumference?
Both are used. US sizes are tied to the inner diameter, while the ISO 8653 standard defines the EU size as the inner circumference in millimetres.
What ring size is a 2 inch finger circumference?
Two inches is 50.8 mm, which works out to about size 5.6. Choose 5.5 or 6 depending on your knuckle and band width.
How accurate is measuring in inches?
A ruler marked in sixteenths changes by 1.59 mm per mark, which is about 0.6 of a ring size. Measure in millimetres when you can.