💍 Ring Size to Diameter Calculator
Select a US ring size to see its standard inner diameter and circumference in millimeters — handy for ring sizing, mandrel shopping, or resizing.
Where the millimetres come from
Diameter is read from a table of 21 US sizes, 3 to 13 in half steps, and the circumference is calculated from it and rounded to 0.1 mm:
circumference = diameter × π (rounded to 1 decimal)Size 7 is 17.3 mm across, so 17.3 × π = 54.3 mm around. The table itself follows the US standard, in which size 0 is 11.63 mm and every whole size adds 1/32 in, or 0.8128 mm, of diameter: 11.63 + 7 × 0.8128 = 17.32 mm. That is why the listed sizes step by 0.4 mm, and why one step - 18.9 to 19.4 between 9 and 9.5 - looks like 0.5. It is rounding, not a real jump.
Measuring and resizing
- Measure round, then divide. Wrap a paper strip snugly at the base of the finger, mark it, measure the length in millimetres and divide by π. 54.3 mm gives 17.3 mm, a size 7.
- European sizes are the circumference. EU 54 is that same 54 mm ring, so the number the tool prints is very nearly the EU size.
- One size up is 2.55 mm of circumference. 0.8128 mm of diameter times π. That is roughly how much metal a resize adds or removes.
- Wide bands fit tighter. Above about 6 mm of band width, go up a half size; comfort-fit interiors run the other way and often need a half size down.
Frequently asked questions
What is the diameter of a size 7 ring?
17.3 mm inside, 54.3 mm around. It is the most commonly stocked women's size in the US.
How do I convert ring circumference to US size?
Divide by π for the diameter, then subtract 11.63 and divide by 0.8128. A 57 mm band is 18.1 mm across, which is size 8.
How much difference is there between ring sizes?
0.8128 mm of diameter per whole size, half that per half size. It is small enough that a half size is worth measuring rather than guessing.