Annulus (Ring) Area Calculator

About this calculator

This calculator finds the area (π × (R² − r²), the area of the outer circle minus the area of the inner circle) and perimeter (the combined circumference of both circles) of an annulus — the flat ring-shaped region between two concentric circles — from its outer and inner radius. It's the geometry behind any shape that's a disc with a circular hole punched out of its center.

It's used to work out the material needed for washers, gaskets, pipes and flanges shown in cross-section, the surface area of a ring-shaped garden bed or fountain border, or the cross-sectional area of tubing where the inner radius represents the hollow bore — any case where a plain circle formula would overcount by including a hole that isn't actually there.