Finding Perimeter
Have you ever walked around the edge of a soccer field or put a frame around a picture? You were following the perimeter! Let's learn how to measure the distance around shapes.
What is Perimeter?
Perimeter = The Distance Around a Shape!
Imagine an ant walking all the way around the outside edge of a shape. The total distance the ant walks is the perimeter. "Peri" means "around" and "meter" means "measure"!
Watch the Ant Walk Around!
Click the button to see how an ant walks the perimeter.
📏 How to Find Perimeter
Finding perimeter is easy! Just follow these steps:
- Look at each side of the shape
- Measure or read the length of each side
- Add all the sides together
- Write your answer with units (inches, feet, cm, etc.)
Adding All the Sides
For any shape, you can find the perimeter by adding up all the side lengths. Let's try some examples!
🔷 Triangle (3 sides)
A triangle with sides: 4 cm, 5 cm, and 6 cm
Perimeter = 4 + 5 + 6 = 15 cm
The perimeter is 15 centimeters!
⬜ Square (4 equal sides)
A square with sides of 5 inches
Perimeter = 5 + 5 + 5 + 5 = 20 inches
Or use the shortcut: 4 x 5 = 20 inches
The perimeter is 20 inches!
▭ Rectangle (2 lengths + 2 widths)
A rectangle: Length = 8 feet, Width = 4 feet
Perimeter = 8 + 4 + 8 + 4 = 24 feet
The perimeter is 24 feet!
Rectangle Shortcuts!
Since rectangles have two pairs of equal sides, we can use special formulas to save time!
Rectangle Perimeter Formulas
This is the same as:
Or even simpler:
⚡ Using the 2L + 2W Formula
Let's find the perimeter of a rectangle that is 10 meters long and 6 meters wide.
Step 1: Write down the formula: P = 2L + 2W
Step 2: Plug in the numbers: P = 2(10) + 2(6)
Step 3: Multiply: P = 20 + 12
Step 4: Add: P = 32
The perimeter is 32 meters!
🎯 Using the 2 x (L + W) Formula
Same rectangle: 10 meters long and 6 meters wide.
Step 1: Add length and width first: 10 + 6 = 16
Step 2: Multiply by 2: 2 x 16 = 32
The perimeter is 32 meters!
Perimeter in the Real World
We use perimeter all the time in everyday life! Here are some examples:
Picture Frames
To know how much wood or material you need to frame a picture, you find the perimeter!
Fencing a Yard
To build a fence around a garden or yard, you need to know the perimeter.
Wrapping Ribbon
To put ribbon around a gift box, you measure the perimeter!
Running Tracks
When you run around a track, you run the perimeter of the field!
Border Decorations
Adding a decorative border around a poster or bulletin board uses perimeter.
Bed Frames
The trim around the edge of a mattress follows the perimeter!
Real World Problem
Sarah wants to put a fence around her rectangular garden. The garden is 12 feet long and 8 feet wide. How many feet of fencing does she need?
Solution:
P = 2L + 2W = 2(12) + 2(8) = 24 + 16 = 40
Sarah needs 40 feet of fencing!
Interactive: Measure the Shapes!
Click a Shape and Find Its Perimeter!
Select a shape above to start!
Draw Your Own Shape!
Click to Place Points and Create a Shape
Click on the grid to place corners of your shape. The perimeter will be calculated automatically!
Practice Problems
Test your perimeter knowledge! Click the correct answer.
Problem 1
A rectangle has a length of 7 cm and a width of 3 cm. What is the perimeter?
Problem 2
A square has sides that are 9 inches each. What is the perimeter?
Problem 3
A triangle has sides of 5 feet, 7 feet, and 8 feet. What is the perimeter?
Problem 4
A rectangular swimming pool is 25 meters long and 10 meters wide. How much rope do you need to go around it once?
Problem 5
Which formula correctly shows how to find the perimeter of a rectangle?
Speed Challenge!
Practice finding perimeters quickly! Type your answer and press Enter.
Quick Perimeter Quiz
Check Your Understanding
What is perimeter?
For a square with side length 6, what is the perimeter?
Maya wants to put ribbon around a rectangular card that is 8 inches by 5 inches. How much ribbon does she need?
What We Learned
Perimeter
The distance all the way around a shape.
Add All Sides
Add every side length to find perimeter.
Rectangle Shortcut
P = 2L + 2W or P = 2 x (L + W)
Square Shortcut
P = 4 x side (all sides equal!)
Next Steps
- Find rectangles around your home and calculate their perimeters
- Practice using both the long way (adding all sides) and shortcuts
- Try measuring in different units (inches, centimeters, feet)
- Remember: perimeter is distance AROUND, area is space INSIDE!