Some polygons are regular, with the lengths of each side equal to the lengths of other sides. Other polygons are irregular, requiring each side to be identified separately. Be certain you have the lengths of all the sides. Also, be certain that each side's length is expressed in identical units.
If necessary, calculate the lengths of any sides where the lengths are not given. Depending on the difficulty and irregularity of the polygon, this can be an easy or difficult process.
For regular polygons, there are formulas to make the addition easier and more predictable. Although these formulas can save some time, they are not necessary. For both regular and irregular polygons, simply add the lengths of all the polygon's sides.
The perimeter of a polygon is expressed in the same measurement units as the sides' lengths. For example, if the sides are all measured in inches, then the perimeter is also expressed in inches.