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Understand and Find Perimeter

Students will learn that perimeter is the total distance around the outside of a polygon. They will find the perimeter of polygons by adding the lengths of all sides, including rectangles, triangles, pentagons, and other polygons.

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Understand and Find Perimeter

Grade 3 · Perimeter of Polygons · question 1 of 3

0 of 12

12 rungs. A star at the top.

ABeach square one square centimeter

Which shape has the shorter distance all the way around?

How it is taught

0:00 of 3:24

Perimeter is the distance around a polygon.

It is the sum of every side length.

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Read it: Walk the four sides

You have a 3 by 5 frame of squares. Counting every edge one by one is slow. The frame shows 15 inside and 16 around.

  1. Walk the four sides of the frame: 5, 5, 3, 3.
  2. Add them: 5 + 5 + 3 + 3 = 16 around.
  3. Keep that apart from the inside count, which is 15.

Counting every tiny edge unit works, but it is slow and you lose track at the corners.

The 15 inside is how much space; the 16 around is how long the fence is.

Opposite sides match: two 5s and two 3s, so you add each pair once.

Add the four sides to walk around; the squares inside answer a different question.

Saying the walk around is 15 because there are 15 squares inside.

The 15 is the inside. Walk the four sides: 5 + 5 + 3 + 3 = 16.

A rectangle is 8 by 3. What is its perimeter? — 22. Walk around: 8 + 3 + 8 + 3 = 22.

Rectangle 6 ft × 4 ft

Here's our rectangular garden. How much fencing do you need to go all the way around?

Grade 3 · How much fencing for our garden?

Here's our garden — it's shaped like a rectangle.

The top side is six feet long.

The right side is four feet long.

The bottom side is also six feet.

And the left side is four feet.

The perimeter is the total distance all the way around the outside of the shape.

To find the perimeter, we add up the lengths of all four sides.

Six plus four is ten.

And six plus four is ten again.

Ten plus ten is twenty.

The perimeter is twenty feet. That's exactly how much fencing we need. Nice work!

What you see

While they climb, your page writes one true line to ask about tonight, and a short note each Sunday.

MayaGrade 3 · Mathematics
Did you know?

A chessboard has 64 squares — share them out between the 8 rows and each row holds exactly 8.

Today’s goal
1 star — one skill mastered, every rung of its ladder climbed
Climbing now
Divide within 100

An example student.

MayaGrade 3 · Mathematics

Maya earned 2 stars this week — Interpret multiplication as equal groups, which took 6 answers over 3 days before its star landed 3 days ago and Multiply by 6, 7, 8 and 9. Next up: Divide within 100.

An example week.

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