Ring size 55
Updated: 14 July 2026
Ring size 55 means an inside circumference of 55 mm. The inside diameter is 17.5 mm, which is about US 7 and UK O. In Europe the number on the ring is the circumference in millimeters, so there is nothing to convert.
Size 55 and the sizes either side
| Size (circumference) | Diameter | US | UK |
|---|---|---|---|
| 53 mm | 16.9 mm | US 6.5 | UK M |
| 54 mm | 17.2 mm | US 7 | UK N |
| 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 |
Where the 17.5 mm diameter comes from
Circumference and diameter are linked by pi. Divide the circumference by 3.1416 and you get the diameter:
55 mm divided by 3.1416 is 17.5 mm.
It works the other way around too. If you measured a ring and read 17.5 mm inside diameter, multiply that by 3.1416 and you are back at 55 mm, which is size 55.
Is 55 really your size?
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.
If 55 does not fit
One size down is 54 mm circumference (17.2 mm diameter), one size up is 56 mm (17.8 mm). One row is one millimeter of circumference, which is roughly a third of a millimeter in diameter.
Whether 55 fits you depends on the ring as well, for example on how wide the band is. Measure again and have a jeweler check the size if the ring is expensive.
Common questions
What does ring size 55 mean?
Ring size 55 means 55 mm inside circumference. The inside diameter is 17.5 mm.
What is ring size 55 in US sizes?
Size 55 is about US 7. US sizes run in half steps, so the conversion rarely lands on a round number.
What diameter goes with ring size 55?
17.5 mm. You get it by dividing the 55 mm circumference by 3.1416.