Add together the length of the four sides of a square to figure its perimeter -- or, if you prefer, multiple the value of one side times four. Multiply two sides together to figure the area. For example, a square that is 2 feet on each side has a perimeter of 2 x 4 feet, or 8 feet, and an area of 2 x 2 feet, or 4 square feet.
Add together the length of the four sides of a rectangle to figure its perimeter -- or, if you prefer, add together its length and width and multiply by two. Calculate the area of a rectangle by multiplying the width times the length. For example, a rectangle that is 2 feet by 4 feet has a perimeter of 2 + 4 + 2 + 4 feet, or 12 feet, and an area of 2 x 4 feet, or 8 square feet.
Calculate the perimeter of a circle, better known as the circumference, by multiplying the radius by two and again by 3.142, the value of pi (circumference = 2 x 3.142 x radius). Multiply the radius by itself and then by pi to calculate the area of the circle (area = pi x radius x radius). For example, a circle with a radius of 3 feet will have a perimeter of 2 x 3.142 x 3, or 18.852 feet, and an area of 3.142 x 3 x 3 or 28.278 square feet.
Figure the perimeter of a triangle by adding up the value of its sides. To find the area of a triangle, multiply its base times its height and divide by two (base + height / 2). For example, a triangle that is 3 feet on each side will have a perimeter of 3 + 3 + 3 or 9 feet and an area of 3 feet x 2.6 feet (its height) or 7.8 square feet, divided by 2 = 3.9 square feet.
