Math Basics: What is Perimeter and How to Find Perimeter

Perimeter is the distance around an object. We can calculate perimeter by adding together the length of all the sides of the object. We are going to discuss how to find the perimeter of polygons, which are shapes with straight sides. There are special rules for finding the distance around round shapes, like circles, which we will talk about another time. Because perimeter is a measure of distance or length, it has to be measured in units – like inches, centimetres, or miles. Units are really important when measuring perimeter, and here’s why: Let’s pretend you need to build a fence around your yard. How much fence would you need to buy? You can add the lengths of all the sides together to find the perimeter, but if you forget to add the units, it still won’t help you! There’s a big difference between 30 inches and 30 meters. When you add the units, you know how long the perimeter is.

Now we’re going to find the perimeter of some more shapes. Let’s start with a triangle! This triangle has three sides with three different lengths: 5 feet, 10 feet, and 8 feet. To find the perimeter of the triangle, all we need to do is add the lengths of the three sides. 5 + 10 + 8 = 23. 23 isn’t the answer, though. We need to add the units – feet, in this case – so we know how long the perimeter is. The perimeter of this triangle is 23 feet. That was pretty easy, right? Let’s try another shape. This time we have a rectangle. Both of the short sides are 2 cm long. Both of the long sides are 4 cm long. To find the perimeter of the rectangle, we add up the lengths of all of the sides. 2 + 4 + 2 + 4 = 12. Since the sides are measured in centimetres, the answer has to be measured in centimetres, too. The perimeter of this rectangle is 12 cm. Next up we have a square. A square has four sides, and all of the sides are the same length. Because the sides are the same length – 5 meters – we can find the perimeter in a different way. We could just add them up – 5 + 5 + 5 + 5 – or, we could use multiplication as a shortcut. Either way, the answer is the same. 5 x 4 = 20, and the perimeter of this square is 20 meters. This would work for any polygon where all the sides are the same length. Just count how many sides there are, and multiply the length of one side by the number of sides. Let’s try another one! This is a star. It has ten sides. Each side is 3 inches long. We could just add up the sides, but it’s much faster to multiply! 3 inches x 10 sides equals the perimeter of this shape: 30 inches. Now that you know how to find the perimeter of an object, grab a ruler and practice measuring the perimeter of things around you!