Definition: The area between two coplanar
concentric circles. A flat ring-shaped object.

Try this Drag either orange dot to resize the circles. The yellow area between them is an annulus.

An annulus is a flat ring shaped object, much like the throw-ring shown on the right. One way to think of it is a circular disk with a circular hole in it. The outer and inner circles that define the ring are concentric (share a common center point).

The dimensions of an annulus are defined by the two radii R2, R1 in the figure above, which are the radii of the outer ring and the inner 'hole' respectively.

The area of the annulus can be found by subtracting the area of the 'hole' from the area of the overall disk. See Area of an Annulus

The adjective form is 'annular'. So for example the ring on the right could be called an 'annular plastic ring'.

- Circle definition
- Radius of a circle
- Diameter of a circle
- Circumference of a circle
- Parts of a circle (diagram)
- Semicircle definition
- Tangent
- Secant
- Chord
- Intersecting chords theorem
- Intersecting secant lengths theorem
- Intersecting secant angles theorem
- Area of a circle
- Concentric circles
- Annulus
- Area of an annulus
- Sector of a circle
- Area of a circle sector
- Segment of a circle
- Area of a circle segment (given central angle)
- Area of a circle segment (given segment height)

- Basic Equation of a Circle (Center at origin)
- General Equation of a Circle (Center anywhere)
- Parametric Equation of a Circle

- Arc
- Arc length
- Arc angle measure
- Adjacent arcs
- Major/minor arcs
- Intercepted Arc
- Sector of a circle
- Radius of an arc or segment, given height/width
- Sagitta - height of an arc or segment

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