Annulus (Ring) Area Calculator
Enter the outer and inner radius of an annulus, the flat ring between two concentric circles, to find its area, both circumferences and its width.
How to find the area of an annulus
- Enter the outer radius, the larger circle.
- Enter the inner radius, the hole in the middle.
- Read the ring area, both circumferences and the width instantly.
Examples
Outer 5, inner 3
outer radius = 5, inner radius = 3
area = 50.2655, width = 2, outer circumference = 31.4159
Outer 10, inner 6
outer radius = 10, inner radius = 6
area = 201.0619, width = 4, inner circumference = 37.6991
Frequently asked questions
What is an annulus?
An annulus is the flat ring left between two circles that share the same centre. A washer or a CD is a familiar example.
What is the formula for the area of an annulus?
The area equals pi times the outer radius squared minus the inner radius squared. It is the big circle area minus the hole.
What is the width of the ring?
The width is the outer radius minus the inner radius. It is the straight distance across the solid part of the ring.
Why must the outer radius be larger than the inner radius?
Otherwise there is no ring. The outer radius has to be strictly greater than the inner radius, which must itself be positive.
What units does this use?
It is unit free. Enter both radii in the same unit and the area is in square units while the circumferences are in the same length unit.
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