US ring size to mm
Enter a US size to see its real dimensions and compare them with a ring you own.
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How to use it
- Enter us size.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Diameter (in) = 0.458 + 0.032 × size, diameter (mm) = diameter (in) × 25.4, circumference = π × diameter.
Example
Inputs: US size = 7. Result: Inner diameter: 17.32 mm; Inner circumference: 54.42 mm; Inner diameter in inches: 0.682 in.
Limits and caveats
- These are the defined dimensions; finished rings can be off by a fraction of a millimeter.
Questions about this tool
What is the diameter of US size 7?
Its inner diameter is 17.32 mm (0.682 in) and its circumference 54.42 mm.
How much does each size add?
Each whole size adds 0.032 in (0.8128 mm) of diameter; a quarter size about 0.2 mm.
How do I measure a ring I own?
Measure the inside diameter edge to edge with a ruler or caliper and compare.