The radius of a circle can be calculated by rearranging the formula for computing diameter or area or perimeter. Hence we have the following formulas. We choose the right one according to what parameters are known in the circle.\begin{aligned} d &= 2r \quad &&\Rightarrow \quad r &&= \frac{d}{2} \\ A &= \pi r^2 \quad &&\Rightarrow \quad r&&= \sqrt{\frac{A}{\pi}} \\ p &= 2 \pi r \quad &&\Rightarrow \quad r &&= \frac{p}{2 \pi} \end{aligned}