If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content

3rd grade

Course: 3rd grade > Unit 11

Lesson 1: Perimeter
Math
3rd grade
Perimeter
Perimeter
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice

Perimeter review

Google Classroom
Review the basics of perimeter and try some practice problems.

What is perimeter?

Perimeter is the distance around the outside of a two-dimensional shape.
The perimeter of the shapes below is shown in blue.
A hexagon.
A triangle.
A four-sided figure.
Want to learn more about the concept of perimeter? Check out this video.

How do we calculate the perimeter of a shape?

To find the perimeter of a shape, we add all of the shape's side lengths.

Example 1:

What is the perimeter of the trapezoid?
A trapezoid with sides labeled as follows: 18 meters, 8 meters, 10 meters, and 10 meters.
We can find the perimeter by adding all of the side lengths.
A trapezoid with sides labeled as follows in different colors: 18 meters in blue, 8 meters in green, 10 meters in purple and 10 meters in red.
Perimeter=18+8+10+10
Perimeter=46 m

Example 2:

What is the perimeter of the regular pentagon?
A pentagon with sides of equal length. One side is labeled 9 centimeters.
We can find the perimeter by adding all of the side lengths.
All sides of a regular pentagon are the same length.
A pentagon with sides of equal length. All sides are labeled 9 centimeters..
Perimeter=9+9+9+9+9
Perimeter=45 cm
Want to learn more about calculating perimeter? Check out this video.

Practice set 1: Calculate perimeter

Problem 1A
What is the perimeter of the triangle?
  • Your answer should be
  • an integer, like 6
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
  • an exact decimal, like 0.75
  • a multiple of pi, like 12 pi or 2/3 pi
centimeters
A triangle with three sides labeled as follows: 8 centimeters, 6 centimeters, and 10 centimeters.

Want to try more problems like this? Check out these exercises:
Find perimeter when given side lengths
Find perimeter by counting unit squares

Practice set 2: Challenge items

Problem 2A
The perimeter of the figure is 50 cm.
What is the length of the missing side?
  • Your answer should be
  • an integer, like 6
  • an exact decimal, like 0.75
  • a simplified proper fraction, like 3/5
  • a simplified improper fraction, like 7/4
  • a mixed number, like 1 3/4
cm
A six-sided polygon with the sides labeled as follows: 14 centimeters, 10 centimeters, 4 centimeters, 8 centimeters, 8 centimeters and one side with question mark centimeters.

Want to try more problems like this? Check out these exercises:
Find a missing side length when given perimeter
Perimeter word problems

Want to join the conversation?

Log in