Ring diameter 18 mm
Updated: 14 July 2026
An inside diameter of 18 mm matches European ring size 57 in this chart. 18 mm times 3.1416 is about 56.5 mm inside circumference, and in Europe that circumference in millimeters is the size. The closest row in the chart is 57 with 18.1 mm diameter.
18 mm in the chart
| Size (circumference) | Diameter | US | UK |
|---|---|---|---|
| 55 mm | 17.5 mm | US 7 | UK O |
| 56 mm | 17.8 mm | US 7.5 | UK P |
| 57 mm | 18.1 mm | US 8 | UK Q |
| 58 mm | 18.5 mm | US 8.5 | UK Q |
| 59 mm | 18.8 mm | US 9 | UK R |
From diameter to size
The size is the inside circumference, but what you usually measure is the inside diameter. Pi connects the two:
- Multiply the diameter by 3.1416. 18 mm times 3.1416 is 56.5 mm.
- That result is the circumference, which is the European size.
- Round to the nearest whole size. Here that is 57.
Measure the diameter from inside edge to inside edge, not across the band. Half a millimeter off ends up as one or two sizes.
How to measure properly
These are the reliable ways to measure:
- Take a ring that already fits the right finger, lay it on a ruler and measure the inside diameter from edge to edge.
- Or use a ring sizer, a set of sample rings you try on one by one.
- Or have a jeweler measure your finger.
The string method (wrap a string around the finger and measure its length) is a fallback for when none of the above is possible. It gives you a rough estimate. Measure the finger you want to wear the ring on and remember that the ring also has to pass your knuckle.
The values either side
17 mm diameter is size 53, 19 mm is size 60. One millimeter of diameter is a good three millimeters of circumference, so about three sizes. That is why it pays to measure twice.
Common questions
What ring size is 18 mm diameter?
Size 57. 18 mm times 3.1416 is about 56.5 mm inside circumference, and the circumference is the European size.
How do I convert diameter into circumference?
Multiply the diameter by 3.1416. 18 mm becomes 56.5 mm of circumference.
What is 18 mm diameter in US sizes?
That is about US 8 and UK Q.