A perimeter, an external measurement of a closed two-dimensional shape, depends on the number and measurements of that shape's sides. Triangles, squares, rectangles, polygons and circles are common two-dimensional shapes that use simple methods for perimeter calculation. Determining the perimeter helps in shape identification, and the perimeter measure can be employed in other calculations.

Add the three sides of a triangle to find the perimeter. For example, if the three sides of a triangle measure 2, 2 and 1.5 inches, the perimeter equals 5.5 inches.

Multiply one side of a square by 4 to find the perimeter. For example, if one side measures 2 inches, then 2 inches multiplied by 4 equals a perimeter of 8 inches.

Multiply the length of a rectangle by 2, multiply the width by 2 and then add them to find the perimeter. For example, if the width of the rectangle is 1 inch and the length is 2 inches, multiplying 1 inch by 2 results in 2 inches, multiplying 2 inches by 2 results in 4 inches, and adding the two numbers results in a perimeter of 6 inches.

Multiply the length of one side of a regular polygon by the number of sides to find its perimeter. Regular polygons have identically sized sides. For example, if the shape is a regular pentagon, which has 5 sides, with a side length of 4 inches, then multiplying 4 inches by 5 results in a perimeter of 20 inches.

Measure each side of an irregular polygon and add the sides to find the perimeter. Irregular polygons have sides of different lengths. An irregular hexagon has six sides that, in this example, measure 3, 3, 4, 5, 2 and 2.5 inches. Adding these sides results in a perimeter of 19.5 inches.

Measure a circle's diameter -- the distance between two opposite points on the circle's perimeter -- and multiply that measurement by pi, a math constant valued at approximately 3.142, to find the circle's perimeter, which is commonly called the circumference. For example, a diameter of 10 inches multiplied by pi produces a circumference of approximately 31.42 inches.