Perimeter
Learn what perimeter means and how to calculate the distance around a shape.
What Is Perimeter?
The perimeter is the total distance around the outside edge of a shape. Imagine an ant walking all the way around the edge of a shape. The distance the ant walks is the perimeter!
Walking Around the Edge
Perimeter = 8 + 5 + 8 + 5
Perimeter = 26 cm
Adding All Sides Together
To find the perimeter, you add up the length of every side of the shape. It works for any shape: triangles, rectangles, pentagons, and more!
Triangle
5 + 5 + 6 = 16 cm
Square
4 + 4 + 4 + 4 = 16 cm
Or: 4 × 4 = 16 cm
Irregular Shape (Pentagon)
6 + 8 + 14 + 8 + 13 = 49 cm
Perimeter in Real Life
We use perimeter all the time in real life! Here are some examples:
Fencing a Garden
If you want to put a fence around a garden, you need to know the perimeter to buy enough fencing.
Garden: 10m long, 6m wide
Fence needed: 10 + 6 + 10 + 6 = 32m
Ribbon Around a Card
Wrapping ribbon around the edge of a birthday card? That is the perimeter of the card!
Card: 15cm tall, 10cm wide
Ribbon needed: 15 + 10 + 15 + 10 = 50cm
Running Track
Running around the edge of a field? You are running the perimeter!
Picture Frame
The frame around a picture goes all the way around the edge, which is the perimeter.
Key Vocabulary
Perimeter
The total distance around the outside of a shape.
Side
One of the straight edges of a shape. A rectangle has 4 sides.
Length
How long something is. We measure length in cm, m, etc.
Rectangle
A shape with 4 sides and 4 right angles. Opposite sides are equal.
Worked Examples
Find the perimeter of a rectangle with sides 7 cm and 3 cm.
Step 1: A rectangle has two long sides (7 cm) and two short sides (3 cm).
Step 2: Add all sides: 7 + 3 + 7 + 3
Answer: Perimeter = 20 cm
Find the perimeter of a square with sides of 9 cm.
Step 1: All 4 sides of a square are equal: 9 cm each.
Step 2: 9 + 9 + 9 + 9 = 36, or 4 × 9 = 36
Answer: Perimeter = 36 cm
A triangle has sides of 4 cm, 6 cm, and 5 cm. What is its perimeter?
Step 1: Add all three sides: 4 + 6 + 5
Answer: Perimeter = 15 cm
Knowledge Check
Calculate the perimeter and select the correct answer.
Question 1
A rectangle has a length of 10 cm and a width of 4 cm. What is the perimeter?
Question 2
A square has sides of 6 cm. What is the perimeter?
Question 3
A triangle has sides of 3 cm, 4 cm, and 5 cm. What is the perimeter?
Question 4
A farmer wants to put a fence around a rectangular field. The field is 12 m long and 8 m wide. How many metres of fencing does he need?
Question 5
A shape has a perimeter of 20 cm. It is a square. How long is each side?
Key Concepts Summary
- ● Perimeter = the total distance around the outside of a shape.
- ● Find the perimeter by adding up all the side lengths.
- ● For a square: perimeter = 4 × side length.
- ● For a rectangle: perimeter = length + width + length + width.
- ● Perimeter is used in real life: fences, frames, paths, and more!