In a regular polygon, where P is the perimeter and n is the number of sides:

`P^2/(4n*tan(180/n))`

Notes

In case future edits cause failure to the equation, here is the original:

(P^2) / (4n*tan(180/n))

I've tested it and I know it works.

However, there has been an issue concerning degrees/ radians. This equation is suited for degrees and it has been modified to work using radians.