An annulus is the area between two circles that share the same centre.
About this calculator
An annulus is the area between two circles that share the same centre. Subtract the smaller circle from the larger one: A = π(R² − r²).
Example: outer radius 5 and inner radius 3 gives area π(25 − 9) = 16π ≈ 50.27 square units. Ring width is R − r = 2.
Use this for flat washers, pipe cross-sections, and track layouts. Pair with [[tube]] for hollow cylinders in 3D, [[circle]] for a full disk, and [[cylinder]] when the ring has height.
Common uses
- Flat washer or gasket area from OD and ID
- Pipe wall cross-section when only radii are known
- Running track lane area between two radii
- Ring-shaped garden bed material estimates
Frequently asked questions
What is the area of an annulus with outer radius 5 and inner radius 3?
A = π(5² − 3²) = π(25 − 9) = 16π ≈ 50.27 square units. Ring width = 2 units.
Can the inner radius be zero?
Yes. When r = 0 the annulus is a full circle with area πR². The calculator treats r = 0 as a solid disk.
How is annulus area different from circumference?
Area is measured in square units (π(R² − r²)). Circumference is a length: outer 2πR plus inner 2πr for both ring edges.
What if inner radius equals outer radius?
The ring has zero width and zero area. Inner radius must stay strictly smaller than outer radius for a non-zero annulus.