Read the lengths of all sides of the shape for which you want to find perimeter. Check that you have all the lengths. Sometimes math textbook writers and test writers like to leave out a number or two in advanced problems. If you are missing a length, compute it. This is almost always a subtraction problem. After figuring out the missing number, write it on the problem.
Add the lengths of all of the sides. "Walk" yourself around the shape with a pencil tip, touching each side as you add. The sum of all the sides of the shape is the shape's perimeter. This method works for every shape except circles.
Apply multiplication as a shortcut to find the perimeter of only an equilateral triangle or a square. Multiply the length of one side by three if the shape is an equilateral triangle; the result is the equilateral triangle's perimeter. Multiply the length of one side by four if the shape is a square; the result is the square's perimeter. You could add every side of an equilateral triangle or a square to find its perimeter, but using the shortcut for these two shapes makes common perimeter math problems faster and easier to solve.