Sector area
From Latin: sector "a cutter"
Definition: The number of square units it takes to exactly fill a sector of a circle.
Try this Drag one of the orange dots that define the endpoints of the sector. The sector area is recalculated as you drag.

What the formulae are doing is taking the area of the whole circle, and then taking a fraction of that depending on what fraction of the circle the sector fills. So for example, if the central angle was 90°, then the sector would have an area equal to one quarter of the whole circle.

If you know the central angle

C  is the central angle in degrees
r  is the radius of the circle of which the sector is part.
π  is Pi, approximately 3.142

This is the method used in the animation above.

If you know the arc length

L  is the arc length.
R  is the radius of the circle of which the sector is part.

Sector area is proportional to arc length

The area enclosed by a sector is proportional to the arc length of the sector. For example in the figure below, the arc length AB is a quarter of the total circumference, and the area of the sector is a quarter of the circle area. Similarly below, the arc length is half the circumference, and the area id half the total circle. You can experiment with other proportions in the applet at the top of the page.
While you are here..

... I have a small favor to ask. Over the years we have used advertising to support the site so it can remain free for everyone. However, advertising revenue is falling and I have always hated the ads. So, would you go to Patreon and become a patron of the site? When we reach the goal I will remove all advertising from the site.

It only takes a minute and any amount would be greatly appreciated. Thank you for considering it!   – John Page

Become a patron of the site at

Other circle topics


Equations of a circle

Angles in a circle