Quick answer: Ring size is based on the inside circumference of the ring. A US size 7 is 5.44 cm (2.14 inches) around; a size 10 is 6.21 cm (2.44 inches). To convert, multiply cm by 10 or inches by 25.4 to get millimeters, then match it to the chart below. Most adult rings fall between about 4.9 cm and 6.7 cm (US 5 to 12).
Ring Size Chart in CM and Inches
Find your finger’s circumference in the second or third column, or the inside diameter of a ring that already fits in the last two. Both describe the same size.
| US size | Circumference (cm) | Circumference (inches) | Inside diameter (mm) | Inside diameter (inches) |
| 3 | 4.42 cm | 1.74 in | 14.1 mm | 0.56 in |
| 3.5 | 4.55 cm | 1.79 in | 14.5 mm | 0.57 in |
| 4 | 4.68 cm | 1.84 in | 14.9 mm | 0.59 in |
| 4.5 | 4.80 cm | 1.89 in | 15.3 mm | 0.60 in |
| 5 | 4.93 cm | 1.94 in | 15.7 mm | 0.62 in |
| 5.5 | 5.06 cm | 1.99 in | 16.1 mm | 0.63 in |
| 6 | 5.19 cm | 2.04 in | 16.5 mm | 0.65 in |
| 6.5 | 5.31 cm | 2.09 in | 16.9 mm | 0.67 in |
| 7 | 5.44 cm | 2.14 in | 17.3 mm | 0.68 in |
| 7.5 | 5.57 cm | 2.19 in | 17.7 mm | 0.70 in |
| 8 | 5.70 cm | 2.24 in | 18.1 mm | 0.71 in |
| 8.5 | 5.83 cm | 2.30 in | 18.5 mm | 0.73 in |
| 9 | 5.95 cm | 2.34 in | 18.9 mm | 0.74 in |
| 9.5 | 6.08 cm | 2.39 in | 19.3 mm | 0.76 in |
| 10 | 6.21 cm | 2.44 in | 19.7 mm | 0.78 in |
| 10.5 | 6.34 cm | 2.50 in | 20.1 mm | 0.79 in |
| 11 | 6.46 cm | 2.54 in | 20.5 mm | 0.81 in |
| 11.5 | 6.59 cm | 2.59 in | 20.9 mm | 0.82 in |
| 12 | 6.72 cm | 2.65 in | 21.3 mm | 0.84 in |
| 12.5 | 6.85 cm | 2.70 in | 21.7 mm | 0.85 in |
| 13 | 6.97 cm | 2.74 in | 22.1 mm | 0.87 in |
Every value here comes from the same millimeter measurements as our US ring size chart, just expressed in centimeters and inches. Tiffany & Co.’s published size guide lists matching millimeter figures for the same sizes, such as 51.9 mm for a size 6 and 54.4 mm for a size 7.
Quick Conversions: 5 cm to 7.5 cm and 2 to 3 Inches
These are the measurements people search for most. Every one of them assumes you measured the circumference, meaning the distance around the finger, not across it.
| Your measurement | In millimeters | Closest US size |
| 5 cm | 50 mm | About 5.5 (between 5 and 5.5) |
| 5.25 cm | 52.5 mm | Between 6 and 6.5 |
| 5.5 cm | 55 mm | Between 7 and 7.5 |
| 6 cm | 60 mm | About 9 (between 9 and 9.5) |
| 6.25 cm | 62.5 mm | About 10 |
| 6.5 cm | 65 mm | About 11 |
| 7 cm | 70 mm | 13 |
| 7.5 cm | 75 mm | Beyond 13 — roughly 15 (approximate) |
| 2 inches | 50.8 mm | About 5.5 |
| 2.25 inches | 57.2 mm | About 8 |
| 2.5 inches | 63.5 mm | About 10.5 |
| 2.75 inches | 69.9 mm | 13 |
| 3 inches | 76.2 mm | Beyond 13 — roughly 15.5 (approximate) |
Sizes beyond 13 are less standardized, so the last row of each group is approximate. See our guide to extra large and petite ring sizes for more.
How to Convert CM or Inches to Ring Size
- Convert your measurement to millimeters. Centimeters multiplied by 10 gives millimeters. Inches multiplied by 25.4 gives millimeters.
- Find that number in the circumference column of any ring size chart. For example, 5.5 cm is 55 mm, which sits between a 7 (54.4 mm) and a 7.5 (55.7 mm).
- If you’re between two sizes, take the larger one. Tiffany’s guide gives the same advice.
If you only have a diameter, multiply it by 3.1416 (pi) to get the circumference. A 17.3 mm diameter times pi is 54.4 mm, which is a US 7. Our existing-ring tool does that conversion for you.
Circumference vs. Diameter: Don’t Mix Them Up
This is the most common mistake with a cm or inch chart. Wrapping string or tape around your finger gives you circumference (the distance around). Laying a ring on a ruler and measuring across it gives you diameter (the distance across). The two columns are about 3.14 times apart, so matching a 1.7 cm diameter to the circumference column would give you a size far too small.
- Measured around a finger or with a tape: use the circumference columns.
- Measured straight across the inside of a ring: use the diameter columns.
Reading a Tape Measure in Fractions of an Inch
Most tape measures mark inches in sixteenths rather than decimals. These whole sizes convert cleanly to the nearest sixteenth:
- US 5: about 1 15/16 inches
- US 6: about 2 1/16 inches
- US 7: about 2 1/8 inches
- US 8: about 2 1/4 inches
- US 10: about 2 7/16 inches
- US 11: about 2 9/16 inches
- US 12: about 2 5/8 inches
- US 13: about 2 3/4 inches
A sixteenth of an inch is about 1.6 mm, which is more than a half size. That’s why a tape measure marked in millimeters is more accurate than one marked only in fractions.
What If You’re Between Sizes?
Size up. A slightly loose ring is easier to fix than a tight one, as our resizing guide explains. For a wide band, you may need to go up a bit more; our wide band sizing guide covers how much.
How to Take the Measurement
For step-by-step instructions, see our guide to measuring ring size with a tape measure or ruler. You can also print our calibrated free ring sizer. For the general overview, see our complete measuring guide. Measure later in the day, since temperature and time of day change finger size.
Frequently Asked Questions
What is a size 7 ring in cm?
A US size 7 has an inside circumference of 5.44 cm and an inside diameter of 1.73 cm (17.3 mm). In inches that’s 2.14 inches around and 0.68 inches across.
What ring size is 6 cm?
If you mean 6 cm around the finger, that’s 60 mm, about a US 9 (59.5 mm) and just under a 9.5 (60.8 mm).
What ring size is 6.5 cm?
A 6.5 cm circumference is 65 mm, which is closest to a US 11 (64.6 mm).
What ring size is 2.5 inches?
2.5 inches is 63.5 mm, which is about a US 10.5 (63.4 mm).
Is ring size measured in cm or mm?
Millimeters. Ring charts are published in mm, so cm and inch measurements get converted first. Multiply cm by 10 or inches by 25.4.
Does the same chart work for men and women?
Yes. It’s one scale. See our men’s ring size chart and average ring size for women for where each group typically falls.
