Millimetres to UK ring size converter
A millimetre measurement of inner circumference converts neatly to a UK letter, because the UK scale is defined in millimetres (1.25 mm per letter from 40 mm at size C). About 51.8 mm is a UK L½ and 54.4 mm is a UK N½. Enter your measurement below.
Quick answer
A mm 54.4 is N½ in UK (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 → UK at a glance
- 46.7 mm = H½
- 49.3 mm = J½
- 51.8 mm = L½
- 54.4 mm = N½
- 56.9 mm = P½
- 59.5 mm = R½
- 62.0 mm = T½
- 64.6 mm = V½
mm to UK ring size chart
| mmCircumference | UKUK / AU | mmDiameter |
|---|---|---|
| 46.7 | H½ | 14.9 |
| 48.0 | I½ | 15.3 |
| 49.3 | J½ | 15.7 |
| 50.5 | K½ | 16.1 |
| 51.8 | L½ | 16.5 |
| 53.1 | M½ | 16.9 |
| 54.4 | N½ | 17.3 |
| 55.6 | O½ | 17.7 |
| 56.9 | P½ | 18.1 |
| 58.2 | Q½ | 18.5 |
| 59.5 | R½ | 18.9 |
| 60.7 | S½ | 19.3 |
| 62.0 | T½ | 19.7 |
| 63.3 | U½ | 20.1 |
| 64.6 | V½ | 20.5 |
| 65.8 | W½ | 21.0 |
| 67.1 | X½ | 21.4 |
The UK scale was built from millimetres
You are converting into a system that is already defined in the units you are holding, which makes this one of the cleanest conversions on the site. Size C is fixed at 40 mm of inner circumference, and each letter adds 1.25 mm.
So a 50 mm circumference sits eight letters above C, which lands on K. A 54.4 mm measurement lands on N½. There is no approximation involved beyond rounding to the nearest half letter.
Circumference or diameter, and why it matters here
The letters are defined against circumference, the measurement around the inside of the band. If you measured straight across a ring instead, that is the diameter and it needs multiplying by 3.1416 first.
The scale of the error is worth understanding. Feeding a 17 mm diameter in as though it were a circumference lands far below the bottom of the letter scale, so the converter returns no valid size at all rather than a wrong one. If you get a dash instead of a letter, you have almost certainly mixed up the two measurements rather than mis-measured.
Getting a measurement you can trust
A steel ruler with half millimetre gradations is enough for an existing ring, though a cheap digital caliper is better and removes the guesswork. Measure the inside opening at its widest point, and take the largest of two or three readings.
Half a UK letter is only about 0.6 mm, so small errors change the answer. If the ring is for an occasion where it cannot be resized, use your measurement as a starting point and confirm it with a jeweller, who will check it on a mandrel in a few seconds.
Common questions
How do I convert a mm ring size to UK?
Enter your mm size in the converter above and read the UK 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 UK?
A mm 54.4 is N½ in UK. That ring has an inner circumference of about 54.4 mm.
Related conversions
Or see the full ring size chart.