Annulus Area Calculator
Calculate the area of an annulus (a flat ring shape) from its outer and inner radius.
What an annulus is
An annulus is the flat, ring-shaped region between two concentric circles (circles sharing the same center) — think of a washer, a donut outline, or a circular track. This calculator finds its area from the outer radius (R) and inner radius (r).
The formula
Area = π · (R² − r²), which is simply the area of the larger circle minus the area of the smaller circle cut out of its middle. The calculator also reports the ring’s width (R − r) and the circumference of both the outer and inner circles.
Note: if the inner radius entered is larger than the outer radius, the calculator automatically swaps them so the result stays valid. This tool is for general math and educational purposes.