Finding the Perimeter of Polygons

I have learned that you don't have to use a ruler or a meter stick to find the perimeter. You can add or multiply to get the perimeter. For example: 2 sides of a square are 3cm, and 2 sides are 4 cm, then you add the 4 sides together, and you get the perimeter of the square. But, usually, you have to mesure the polygon to find the area.