Ring size from inner diameter
Put a ruler across a ring that fits and measure inside edge to inside edge, then enter it here.
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Processing and privacy: The result is calculated inside your browser and your data is not sent to any server.
How to use it
- Enter inner diameter.
- 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
Circumference = π × diameter, then each system: US = (diameter ÷ 25.4 − 0.458) ÷ 0.032, EU = circumference, Japanese = (diameter − 13) × 3 + 1.
Example
Inputs: Inner diameter = 17.3 mm. Result: US size: 7; UK size: N½; EU size (ISO): 54.
Limits and caveats
- Half a millimeter of error in the diameter shifts the result by about half a US size, so use a caliper if you can.
- Sizes are rounded to the nearest step of each system, and shop mandrels can differ slightly.
Questions about this tool
My ring is 17.3 mm across. What size is it?
That is US 7, UK N½ and EU 54.
Where exactly do I measure?
Across the widest inside point, between the inner edges, not the outer ones.
What is the Italian/Swiss size?
The circumference in mm minus 40, so 54 mm around is size 14.