Annulus (ring) area calculator
Enter the outer and inner radius to get the ring area, its width and the area of each circle.
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How to use it
- Enter outer radius, inner radius and length unit.
- 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
Ring area = π × (R² − r²) and ring width = R − r, where R is the outer and r the inner radius.
Example
Inputs: Outer radius = 5, Inner radius = 3, Length unit = Centimeters (cm). Result: Ring area: 50.2655 cm²; Ring width: 2 cm; Outer circle area: 78.5398 cm².
Limits and caveats
- It assumes both circles share the same center.
- If you have diameters, halve them before entering.
Questions about this tool
What is the formula for the area of an annulus?
Subtract the inner circle from the outer: π × (R² − r²). A ring with radii 5 and 3 has an area of 16π, about 50.27.
How do I find the cross-section area of a pipe?
Use the pipe's outer radius as R, and the outer radius minus the wall thickness as r.
Is there another way to write the formula?
Yes: π × (R + r) × (R − r), which is the mean circumference times the ring width.