Area of a circle

$$A=\pi r^2$$

A = area

π = 3.14159265...

r = radius

Other formulas
If you only know the diameter

Diameter = 2 * radius

If you only know the circumference

Circumference = π * diameter

So: $$A=\pi \frac{d}{2}^2$$

$$A=\frac{C}{2}^2$$