Perimeter of Polygons

🎯 What Will We Learn Today?

How much fence do you need around a garden? That's the perimeter! Today we'll learn how to calculate the perimeter of different polygons — triangles, rectangles, and more.

📚 What Is Perimeter?

The perimeter is the total distance around a shape. To find it, just add up all the side lengths!

If you walked all the way around a shape, the distance you walked is the perimeter.

📐 Perimeter Formulas for Common Shapes

Triangle

Perimeter = side₁ + side₂ + side₃

A triangle with sides 5 cm, 7 cm, and 9 cm:
Perimeter = 5 + 7 + 9 = 21 cm

Rectangle

Perimeter = 2 × (Length + Width) = 2L + 2W

A rectangle 8 m long and 3 m wide:
Perimeter = 2 × (8 + 3) = 2 × 11 = 22 m

Square

Perimeter = 4 × side

A square with 6-inch sides:
Perimeter = 4 × 6 = 24 inches

Regular Pentagon（5 equal sides）

Perimeter = 5 × side

Side = 4 cm → Perimeter = 5 × 4 = 20 cm

🎮 Finding Missing Sides

Sometimes you know the perimeter but one side is missing:

Problem: A rectangle has perimeter 30 cm. The length is 10 cm. What's the width?

Perimeter = 2L + 2W = 30

2 × 10 + 2W = 30

20 + 2W = 30

2W = 10 → W = 5 cm

💡 Perimeter in Real Life

Fencing a yard — how many feet of fencing?

Framing a picture — how much frame material?

Running track — one lap around is the perimeter!

Christmas lights — how many feet for the roofline?

📝 Practice Problems

A triangle has sides 6 cm, 8 cm, and 10 cm. What is its perimeter?

A rectangular garden is 12 ft by 5 ft. How much fencing is needed?

A square playground has a perimeter of 36 m. How long is each side?

A regular hexagon has each side = 3 inches. What's the perimeter?

A rectangle has perimeter 24 cm and width 4 cm. Find the length.

🎮 Play a Game!

Practice perimeter skills with Island Chase!