If you have ever been sent to your room for punishment, you might have walked its perimeter out of boredom. The perimeter of an object is the measurement of the borders of its area. Like determining the area of the room, finding its perimeter requires you to measure the length of the walls; but unlike the area, the perimeter is concerned only with the boundaries of the space.
Multiply the length of one wall by 4 to calculate the perimeter of a square room, since all four sides of a square are the same length by definition. Think of the floor as the square. For example, suppose the length of one wall is 20 feet. 20 multiplied by 4 gives a perimeter of 80 feet.
Double the width and length measurements and add them together to calculate the perimeter of a rectangular room. For example, suppose the length of the room is 18 feet and its width is 14 feet. Doubling 18 results in 36 and doubling 14 results in 28. Adding 36 and 28 together equals 64 feet.
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Add the lengths of the three walls together to calculate the perimeter of a triangular room. For example, the lengths of the walls are 15 feet, 20 feet and 20 feet. Adding those lengths together gives a perimeter of 55 feet.
Multiply the diameter of a circular room by pi to calculate its perimeter. The diameter of a circle is the distance from one side to the other, passing through the center. Pi, or π, is a nonrepeating mathematical constant in the form of a decimal that starts out 3.14159. For example, suppose the diameter of the room is 20 feet. Multiplying 20 by π gives a perimeter of 62.832, rounded off.
Add together the lengths of all the walls to calculate the perimeter of any room. For example, suppose a room has two curved walls that measure 18 and 20 feet, and three straight walls that measure 18, 24, and 15 feet. Adding those numbers together results in a perimeter of 95 feet.