Ring size 51
Updated: 14 July 2026
Ring size 51 means an inside circumference of 51 mm. The inside diameter is 16.2 mm, which is about US 5.5 and UK L. In Europe the number on the ring is the circumference in millimeters, so there is nothing to convert.
Size 51 and the sizes either side
| Size (circumference) | Diameter | US | UK |
|---|---|---|---|
| 49 mm | 15.6 mm | US 5 | UK J |
| 50 mm | 15.9 mm | US 5.5 | UK K |
| 51 mm | 16.2 mm | US 5.5 | UK L |
| 52 mm | 16.6 mm | US 6 | UK M |
| 53 mm | 16.9 mm | US 6.5 | UK M |
Where the 16.2 mm diameter comes from
Circumference and diameter are linked by pi. Divide the circumference by 3.1416 and you get the diameter:
51 mm divided by 3.1416 is 16.2 mm.
It works the other way around too. If you measured a ring and read 16.2 mm inside diameter, multiply that by 3.1416 and you are back at 51 mm, which is size 51.
Is 51 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 51 does not fit
One size down is 50 mm circumference (15.9 mm diameter), one size up is 52 mm (16.6 mm). One row is one millimeter of circumference, which is roughly a third of a millimeter in diameter.
Whether 51 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 51 mean?
Ring size 51 means 51 mm inside circumference. The inside diameter is 16.2 mm.
What is ring size 51 in US sizes?
Size 51 is about US 5.5. US sizes run in half steps, so the conversion rarely lands on a round number.
What diameter goes with ring size 51?
16.2 mm. You get it by dividing the 51 mm circumference by 3.1416.