How do you find the area of a circle given the diameter but not the radius

So :

Area of circle = πr²

The diameter is two times the radius so you would divide the diameter by two.

For example if the diameter was 12cm, the radius would be 6cm. So then :
Area = π(6)²
Area = π(36)
Area = 113.04 cm²
This is an estimated answer because π is an irrational number.

The decimal expansion would be .27 repeated