Perimeter of a circle

The perimeter of a circle can be calculated with these two formulas\begin{aligned} p &= 2 \pi r \\ p &= \pi d \end{aligned}with radius $r$ or diameter $d$ known.

If you want to find for example the radius of the circle but only know the perimeter, you can rearrange the formula for $r$ and get what you were looking for.