What Is Circumference?

🎯 What Will We Learn Today?

How long is the edge of a circle? Today we'll learn about circumference — the distance around a circle.

📚 What Is Circumference?

The circumference is the distance around a circle — just like the perimeter is the distance around a square or rectangle. If you could take the edge of a circle and stretch it into a straight line, that line would be the circumference.

Think of wrapping a string around a can or a jar. The length of string you need is the circumference!

🔍 How Circumference Relates to Diameter

Here's something amazing: for every circle in the world, if you divide the circumference by the diameter, you always get the same number — about 3.14. This special number is called pi (π).

Circumference ÷ Diameter = π (always!)

This means: Circumference = π × Diameter

Or using radius: Circumference = 2 × π × Radius

🎮 Try It Yourself!

Activity: Find a round object (a cup, a can, a bottle cap).

Measure its diameter (the distance across the center)

Wrap a string around it to measure its circumference

Divide the circumference by the diameter

You'll get a number close to 3.14!

💡 Key Formulas

C = π × d (circumference = pi × diameter)

C = 2 × π × r (circumference = 2 × pi × radius)

Using π ≈ 3.14:

If diameter = 10 cm → C ≈ 3.14 × 10 = 31.4 cm

If radius = 7 cm → C ≈ 2 × 3.14 × 7 = 43.96 cm

📝 Practice Problems

A circle has a diameter of 8 cm. What is its approximate circumference? (Use π ≈ 3.14)

The radius of a circle is 5 inches. Find the circumference.

If a bicycle wheel has a circumference of 188.4 cm, what is its approximate diameter?

Why does a bigger circle have a bigger circumference?

🎮 Play a Game!

Learn geometry with fun racing games at Grand Prix Multiplication!