Area of a Sector - Formula, Definition, Examples - Cuemath Apply the unitary method to derive the formula of the area of a sector of circle. We know, a complete circle measures 360º. The area of a circle with an angle measuring 360º at the center is given by πr 2, where r is the radius of the circle. If the angle at the center of the circle is 1º, the area of the sector is πr 2 /360º.