Each US size is a fixed step in diameter
US ring sizes are evenly spaced: every whole size adds 0.8128 mm (about 0.032 in) to the inside diameter. Measure across the inside of a ring that fits, convert to millimeters if needed (multiply inches by 25.4), and drop it into the formula. Half sizes fall exactly halfway between.
Inside diameter to US ring size
| Inside diameter (in) | Inside diameter (mm) | US size |
|---|---|---|
| 0.555 in | 14.1 mm | 3 |
| 0.587 in | 14.9 mm | 4 |
| 0.618 in | 15.7 mm | 5 |
| 0.650 in | 16.5 mm | 6 |
| 0.681 in | 17.3 mm | 7 |
| 0.713 in | 18.1 mm | 8 |
| 0.748 in | 19.0 mm | 9 |
| 0.780 in | 19.8 mm | 10 |
| 0.811 in | 20.6 mm | 11 |
| 0.843 in | 21.4 mm | 12 |
| 0.874 in | 22.2 mm | 13 |
To go from your finger instead of a ring, wrap a strip of paper around it, mark where it overlaps, and divide that circumference by π (3.1416) to get the diameter.
Getting the measurement right
- Measure the opening, not the band. The number that matters is the inside hole, straight across the center.
- Fingers change through the day. They are smallest in the morning and can swell 2 to 3 mm by evening. Measure late, and skip cold days.
- US, EU and UK differ. EU size is roughly US + 17 and reads off the diameter in mm; the UK uses letters. A "size 7" abroad is not a US 7.
- Half a size is small. One full size is only 0.8 mm of diameter, so a 0.4 mm measuring error can shift your result by half a size.
Common questions
How do I measure the inside diameter of a ring?
Lay a ring that already fits flat and measure straight across the inside opening, edge to edge through the center. That distance in inches or millimeters maps to a US size. Measure the hole, not the band.
What is the average ring size?
For women the average is around US 6 to 7, an inside diameter of roughly 0.65 to 0.68 inches. For men it is about US 9 to 10, roughly 0.75 to 0.78 inches. Men tend to have wider fingers, so larger sizes are normal.
I am between two sizes. Which do I pick?
Pick the smaller size when buying new, since a jeweler can size a ring up more easily than down. Also measure at the end of the day, when fingers are at their largest, so the ring is not too tight most of the time.
