Centroid of a circle is very easy to determine. Centroid of circle lies at the center of a circle that is also called as the radius of circle from edges of a circle. As shown below.
Examples

Find the center of gravity (centroid) of circular wheel whose diameter is 20cm.
Solution:
Centroid of circular section lies at its center i.e., D/2
R= D/2 = 20/2 = 10cm= Answer

Find the centroid of hollow circular section whose outer diameter is 20cm and 16cm inner diameter.
Solution:
Centroid of circular section lies at D/2 from each side of a circle.
Hence, with respect to outer diameter centroid lies at;
D/2 = 20/2 = 10cm
With respect to inner diameter centroid lies at;
D/2 = 16/2 = 8cm = Answer