Sector Area

A sector is the pie-slice region bounded by two radii and an arc. The same idea from arc length applies to area: a sector is the same fraction of the area of the circle that its arc is of the circumference.

For a sector with central angle θ (degrees) and radius r:

A 90° sector is a quarter of the circle’s area. A 180° sector is a half. The relationship is the same simple fraction every time.