MM to Ring Size: Convert mm, cm or Inches
Measured your finger with a strip of paper or string, or a ring with a ruler? Enter the number to get your US, UK, European and Japanese ring size.
Inside diameter 17.2 mm · circumference 54.0 mm. See your ring at actual size
How to use it
- Around your finger: wrap a thin strip of paper or string around the base of the finger, mark where it overlaps, and measure the length with a ruler. Enter it as the circumference.
- Across a ring: measure the inside of a ring that fits, edge to edge through the center, in millimeters. Enter it as the diameter.
- Between sizes? Go up to the next half size, especially for wide bands.
No ruler at hand? Use the on-screen mm ruler, or skip the measuring and lay a ring on the ring size finder.
Finger circumference in cm to ring size
| Around the finger | US | UK | EU |
|---|---|---|---|
| 4.5 cm (45 mm) | 3¼ | F½ | 45 |
| 4.8 cm (48 mm) | 4½ | I | 48 |
| 5 cm (50 mm) | 5¼ | K | 50 |
| 5.2 cm (52 mm) | 6 | L½ | 52 |
| 5.4 cm (54 mm) | 6¾ | N | 54 |
| 5.6 cm (56 mm) | 7½ | O½ | 56 |
| 5.8 cm (58 mm) | 8½ | Q¼ | 58 |
| 6 cm (60 mm) | 9¼ | S | 60 |
| 6.2 cm (62 mm) | 10 | T½ | 62 |
| 6.5 cm (65 mm) | 11¼ | W | 65 |
| 7 cm (70 mm) | 13 | Z½ | 70 |
Finger circumference in inches to ring size
| Around the finger | US | UK | EU |
|---|---|---|---|
| 1 3/4 in | 3 | F | 44 |
| 2 in | 5½ | K½ | 51 |
| 2 1/8 in | 6¾ | N | 54 |
| 2 1/4 in | 8 | P½ | 57 |
| 2 3/8 in | 9¼ | S | 60 |
| 2 1/2 in | 10½ | U½ | 64 |
| 2 3/4 in | 13 | Z½ | 70 |
Ring inside diameter in mm to ring size
| Inside diameter | US | UK | EU |
|---|---|---|---|
| 15 mm | 4¼ | H½ | 47 |
| 15.5 mm | 4¾ | J | 49 |
| 16 mm | 5½ | K | 50 |
| 16.5 mm | 6 | L½ | 52 |
| 17 mm | 6½ | M½ | 53 |
| 17.5 mm | 7¼ | O | 55 |
| 18 mm | 7¾ | P | 57 |
| 18.5 mm | 8½ | Q¼ | 58 |
| 19 mm | 9 | R½ | 60 |
| 19.5 mm | 9¾ | T | 61 |
| 20 mm | 10¼ | U | 63 |
| 21 mm | 11½ | W½ | 66 |
| 22 mm | 12¾ | Z | 69 |
US sizes are rounded to the nearest quarter size. To see a size drawn at actual size on your screen, use the ring size finder. For the full chart in every system, see the ring size chart.
The math
US ring sizes follow a simple formula: inside diameter (mm) = 11.63 + 0.8128 × size. To go from a circumference, divide it by π (3.1416) to get the diameter first. European sizes (ISO 8653) are simply the circumference in millimeters, and Japanese sizes are about 3 × diameter − 38.
Frequently asked questions
What ring size is 6 cm?
A finger that measures 6 cm (60 mm) around is about a US size 9¼, UK S or European size 60. That's on the larger side for women and typical for men.
What ring size is 7 cm?
7 cm (70 mm) around is about a US size 13, UK Z½, European size 70. It's a large size, often worn on the thumb or by men with larger hands.
What ring size is 2.5 inches?
A finger measuring 2.5 inches (63.5 mm) around is about a US size 10½, UK U½, European size 64.
What ring size is 17 mm?
If 17 mm is the inside diameter of a ring (measured straight across), it's about a US size 6½. If 17 mm is the distance around your finger, it's far too small to be a finger: measure again with a strip of paper, or check whether you measured across instead of around.
Should I measure around my finger or across a ring?
Either works. Around the finger (circumference) is what a paper strip or string gives you, and is usually between 44 and 70 mm for adults. Across a ring (diameter) is what you get with a ruler or calipers on a ring that fits, usually between 14 and 22 mm.