Pronunciation: /ˈæn yə ləs/ Explain

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What is the difference between an annulus and a circle?
Manipulative 1 - Annulus Created with GeoGebra.

An annulus is the region between two concentric circles.[1] The area of the annulus is the difference between the areas of the circles: A = π(r12 - r22) where r1 > r2.

Check box Understanding Check

What is the area of an annulus with an inner radius of 1m and an outer radius of 3m?

Click here for the answer.

Examples of Annulus

A roll of paper towels is an annulus.
Figure 2: Roll of paper towels
A roll of pipe tape is an annulus.
Figure 3: Roll of pipe tape
A pipe is an annulus.
Figure 4: Pipe


1. A pipe has an inner radius of 3 cm. It has an outer radius of 3.5 cm. It is 100 cm long. What is the total volume of the pipe, not including the empty space inside?

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Click here for the answer.

2. The same pipe is made of a metal alloy with a density of 3g/cm3. What is the mass of the pipe in challenge 1?

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  1. Stöcker, K.H.. The Elements of Constructive Geometry, Inductively Presented. pp 44-45. Translated by Noetling, William A.M, C.E.. Silver, Burdett & Company. 1897. Last Accessed 8/6/2018. Buy the book

More Information

  • Annulus. Math Is Fun. 6/19/2018.

Cite this article as:

McAdams, David E. Annulus. 7/10/2018. All Math Words Encyclopedia. Life is a Story Problem LLC.

Image Credits

  • All images and manipulatives are by David McAdams unless otherwise stated. All images by David McAdams are Copyright © Life is a Story Problem LLC and are licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.
  • Roll of Paper Towels: Unknown. Unknown.
  • Roll of pipe tape: Unknown. Unknown.
  • Pipe: Unknown. Unknown.

Revision History

6/14/2018: Removed broken links, changed Geogebra links to work with Geogebra 5, updated license, implemented new markup. (McAdams, David E.)
12/27/2009: Added "Reference" and "More Information". (McAdams, David E.)
12/19/2008: Changed figure 1 to a manipulative. (McAdams, David E.)
6/23/2008: Reorganized content, added challenge and understanding check. (McAdams, David E.)
4/18/2008: Added illustrations. (McAdams, David E.)
4/17/2008: Initial version. (McAdams, David E.)

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This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License