Perimeter: the distance around a shape
In 3rd grade, children learn to find the perimeter of a shape, to work out a missing side length, and to see that rectangles with the same perimeter can have different areas.
By Robert “Bobby” BarnettMath teacher, parent, and founder of Bobby’s Math Club
Try these with your child
These are real questions from our 3rd grade practice. Tap the speaker to hear one read aloud, or the light bulb for a hint, and talk about the answer together.
Question 1 of 3
A rectangular garden has sides that are 5 feet long and 3 feet wide. What is the perimeter of the garden?
- 8 feet
- 15 feet
- 16 feet
Why this matters
Fences, picture frames, and bulletin-board borders all give your child a reason to think about perimeter, the distance around an edge. Comparing it with area helps your child keep the two measurements separate, which matters in 4th grade when they use both to solve real problems. Working out a missing side also builds the backward reasoning that algebra relies on later.
Where kids can get stuck
- Adding only the two side lengths shown, so a 5 by 3 rectangle gets a perimeter of 8 instead of 16.
- Multiplying the sides to find perimeter, mixing it up with area, so a 5 by 3 rectangle gets 15.
- Assuming equal perimeters mean equal areas, so a 7 by 1 rectangle and a 4 by 4 square seem the same size.
How you can help
- Walk around the edge of a room, yard, or table and count your steps, then talk about how the total is the perimeter in steps.
- Use string or yarn to go around a picture frame, book, or placemat, then straighten it and measure the length with a ruler.
- Give your child 12 toothpicks and ask how many different rectangles they can make using all of them, then compare how much space each one covers.
If it’s helpful, you can sign up for free, adaptive practice in Bobby's Math Club.
Questions parents ask
What is the difference between perimeter and area?
Perimeter is the distance around a shape, like a fence around a yard. Area is the space inside, like the grass the fence encloses. Perimeter is measured in units like feet, and area in square units like square feet. A helpful question for your child is whether the problem is about going around or covering.
How does my child find a missing side?
Your child uses the total perimeter and the sides they know. If a triangle has a perimeter of 18 inches and two sides measure 5 and 7 inches, the missing side is 18 - 5 - 7 = 6 inches. For rectangles, they use the fact that opposite sides are equal.
Why is my child comparing rectangles with the same perimeter?
Comparing them shows that perimeter and area measure different things. Rectangles 1 by 7, 2 by 6, 3 by 5, and 4 by 4 all have a perimeter of 16, but their areas are 7, 12, 15, and 16. This kind of exploration helps your child stop mixing up the two measurements and builds reasoning about shapes.
Make learning math fun
Bobby's Math Club turns 3rd grade math practice into something fun to do together.
- Unlimited practice, completely free
- Practice that starts where your child is and grows with them
- You see their progress and what to work on next
Written by Robert “Bobby” Barnett
Bobby is an award-winning math teacher, the author of Meet Every Learner’s Needs, and the father of two elementary-school math learners. He uses Bobby’s Math Club with his own kids a few times every week. Learn more or contact Bobby.
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Join the clubNote for teachers: This skill is covered by Common Core math standard 3.MD.D.8: “Solve real world and mathematical problems involving perimeters of polygons, including finding the perimeter given the side lengths, finding an unknown side length, and exhibiting rectangles with the same perimeter and different areas or with the same area and different perimeters.” Learn how Bobby's Math Club can help your students grow. (Common Core 3.MD.8)