Millimetres to US ring size converter
If you have measured a finger or an existing ring in millimetres, you can convert straight to a US size. Inner circumference maps directly: about 51.8 mm is a US 6 and 54.4 mm is a US 7. If you measured the diameter instead, multiply by π (3.1416) to get circumference, or just use the tool below.
Quick answer
A mm 54.4 is 7 in US (about 54.4 mm inner circumference). Use the converter for your exact size, or the chart below for the full range.
Between two sizes? Size up for a comfortable fit, see how to measure accurately.
mm → US at a glance
- 46.7 mm = 4
- 49.3 mm = 5
- 51.8 mm = 6
- 54.4 mm = 7
- 56.9 mm = 8
- 59.5 mm = 9
- 62.0 mm = 10
- 64.6 mm = 11
mm to US ring size chart
| mmCircumference | USUS / Canada | mmDiameter |
|---|---|---|
| 46.7 | 4 | 14.9 |
| 48.0 | 4½ | 15.3 |
| 49.3 | 5 | 15.7 |
| 50.5 | 5½ | 16.1 |
| 51.8 | 6 | 16.5 |
| 53.1 | 6½ | 16.9 |
| 54.4 | 7 | 17.3 |
| 55.6 | 7½ | 17.7 |
| 56.9 | 8 | 18.1 |
| 58.2 | 8½ | 18.5 |
| 59.5 | 9 | 18.9 |
| 60.7 | 9½ | 19.3 |
| 62.0 | 10 | 19.7 |
| 63.3 | 10½ | 20.1 |
| 64.6 | 11 | 20.5 |
| 65.8 | 11½ | 21.0 |
| 67.1 | 12 | 21.4 |
First, decide which measurement you have
This is the single most common mistake, and it produces answers that are wildly wrong rather than slightly wrong. Circumference is the distance all the way around the inside of the band. Diameter is the distance straight across it. They differ by a factor of about 3.14.
If you wrapped string or paper around a finger, you have a circumference, probably somewhere between 44 and 66 mm. If you laid a ring flat and measured across the opening, you have a diameter, probably between 14 and 21 mm. A number near 17 is a diameter. A number near 54 is a circumference. The converter accepts either, so long as you tell it which.
Measuring a ring you already own
Measuring an existing ring is more reliable than measuring a finger, because a ring holds its shape. Lay it on a flat surface and measure the inside opening at its widest point, in millimetres, using a ruler with fine gradations or a digital caliper.
Take the measurement two or three times and use the largest reading. It is easy to measure slightly off centre, and every off-centre reading is too small. A third of a millimetre of diameter is roughly four tenths of a US size, so the care is worth it.
Measuring a finger instead
Wrap a strip of paper or a piece of non-stretching string around the base of the finger, mark where it overlaps, then lay it flat against a ruler. That length is the circumference. Avoid using thread or elastic, which stretch and will read small.
The strip has to pass over the knuckle, so check it slides off before you trust the number. Measure late in the day and when your hands are warm, since fingers can change by up to half a size between morning and evening.
Common questions
How do I convert a mm ring size to US?
Enter your mm size in the converter above and read the US result, or find your row in the chart below. Every value is calculated from the ring’s inner circumference, so it works in both directions.
What is a mm 54.4 in US?
A mm 54.4 is 7 in US. That ring has an inner circumference of about 54.4 mm.
Related conversions
Or see the full ring size chart.