Annulus (Ring) Area Calculator

Find the area of a ring (annulus) between two circles, plus ring width and both circumferences, from inner and outer radius.

Half the outside diameter.
Half the hole diameter. Must be smaller than R.
Ring area
50.265
Ring width (R − r)
2
Outer circumference
31.416
Inner circumference
18.85

Estimate generated with SortedSums: https://sortedsums.com/annulus-calculator/. For informational purposes only; verify before relying on it.

How it’s calculated

An annulus is the flat ring between two concentric circles. Its area is the big circle minus the hole.

Area = π(R² − r²) = π(R + r)(R − r) Width w = R − r

Example: R = 5 in and r = 3 in gives Area = π(25 − 9) = 16π ≈ 50.27 in² and a ring width of 2 in.

Frequently asked questions

How do I find the area of a ring?

Subtract the area of the inner circle from the outer circle: π(R² − r²). Use radii, not diameters.

What if I only know diameters?

Halve each diameter to get the radii first. A pipe with 10 in outside and 6 in inside diameter has R = 5 in and r = 3 in.

Can I use it for a path around a round pool?

Yes. R is the pool radius plus the path width and r is the pool radius; the result is the path’s surface area.

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Sources

Formulas are taken from the free public references above. Results are provided “as is” for informational and educational purposes only. See our disclaimer.

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