Reading Comprehension · Math
Perimeter
A math lesson explaining perimeter — the distance around an object — and how to calculate it for squares, rectangles, and irregular shapes.
Subject
Reading Comprehension
Grade Level
Grades 3–6
Format
Lesson + Printable PDF
Access
Full Member
What Is Perimeter?
The perimeter of a shape is the total distance around its outside edge. To find the perimeter, you simply add up the lengths of all the sides. Every type of shape — squares, rectangles, triangles, and more — has a perimeter that can be measured and calculated.
A square has four equal sides. To find the perimeter of a square, add the four sides together. If each side measures 4 inches, the perimeter is 4 + 4 + 4 + 4 = 16 inches. You can also think of it as 4 times the length of one side: 4 × 4 = 16 inches. Always include the unit of measurement — inches, feet, centimeters, or meters — at the end of your answer.
A rectangle has two pairs of equal sides. The longer pair is called the length and the shorter pair is called the width. To find the perimeter of a rectangle, add all four sides: length + length + width + width. Another way to write this is (2 × length) + (2 × width) = perimeter. For example, a rectangle that is 15 inches long and 6 inches wide has a perimeter of (2 × 15) + (2 × 6) = 30 + 12 = 42 inches.
Perimeter of Irregular Shapes
Not all shapes are simple squares and rectangles. Irregular shapes — shapes with sides of different lengths — also have perimeters. To find the perimeter of an irregular shape, you use the same method: add up the lengths of all the sides. The more sides the shape has, the more numbers you add, but the process is always the same.
One helpful method for irregular shapes is to count grid squares along each side. If you place a shape on grid paper and count how many squares long each side is, you can add those counts together to get the total perimeter. This makes it easy to find the perimeter of shapes that would be hard to measure with a ruler.
Perimeter has many practical uses in the real world. Knowing the perimeter of a room helps you figure out how much baseboard trim or fencing you need. A family that wants to build a fence around a rectangular yard needs to know the perimeter to determine how many feet of fencing to buy. For instance, a yard that is 36 feet long and 17 feet wide has a perimeter of (2 × 36) + (2 × 17) = 72 + 34 = 106 feet of fencing needed.
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