If $C$ is a circle’s circumference and $r$ is its radius, then $C=2\pi r$. The area of a circle can be found by taking the product of half the circumference and the radius.

If $A$ is the area of the circle, this gives the equation:

$A = \frac12 (2\pi r) \boldcdot r$

This equation can be rewritten as:

$A=\pi r^2$

Remember that when we have $r \boldcdot r$ we can write $r^2$, and we can say “$r$ squared.”

This means that if we know the radius, we can find the area. For example, if a circle has a radius of 10 cm, then its area is about $(3.14) \boldcdot 100$, which is 314 cm².

If we know the diameter, we can figure out the radius, and then we can find the area. For example, if a circle has a diameter of 30 ft, then the radius is 15 ft, and the area is about $(3.14) \boldcdot 225$, which is approximately 707 ft².