Detailed definition
Understanding Annulus
An annulus is the region between two coplanar concentric circles. It looks like a flat ring because the inner circle removes a smaller disk from a larger one without shifting the center.
The geometry of an annulus comes from subtraction. Its area is found by subtracting the area of the inner disk from the area of the outer disk, which is why the two circles must share a center.
This page keeps both circles aligned around the same center so you can see that an annulus is defined by concentric structure, not merely by having one circle inside another.
Key facts
Important ideas to remember
- An annulus is the ring-shaped region between two concentric circles.
- Both circles in an annulus are concentric, meaning they share the same center.
- The annulus has an inner radius and an outer radius, and both matter in measurement problems.
- Annulus area is the difference between two circle areas, not the product or the average of the two radii.
Where it is used
Where annulus shows up
- Use annuli when working with washers, rings, circular tracks, and pipe cross-sections.
- Use them in area problems where one circular region is removed from a larger one.
- Use the concept whenever a circle-based figure has a hole in the middle but keeps the same center.
Common mistakes
What to watch out for
- Do not call the region an annulus if the two circles are not concentric.
- Do not subtract radii and mistake that for the area of the ring.
- Do not confuse the thickness of the annulus with its full area.