Skill: Calculate the circumference of a circle

The circumference is the distance around a circle. Use one of these formulas:

• If you know the radius: $C=2\pi r$
• If you know the diameter: $C=\pi d$

Here, $C$ is circumference, $r$ is radius, and $d$ is diameter. Use $\pi\approx 3.14$ unless your teacher asks you to use a calculator’s $\pi$ button. The answer must be in linear units, such as centimeters or meters.

Example

A circular garden has a diameter of $8$ meters. Find its circumference.

Step 1: Choose the formula.

The diameter is given, so use

$$C=\pi d$$

Step 2: Substitute the known values.

Use $\pi\approx 3.14$ and $d=8$:

$$C\approx 3.14(8)$$

Step 3: Multiply.

$$C\approx 25.12$$

Step 4: Include the units.

The circumference is

$$\boxed{25.12\text{ meters}}$$

This is the distance around the garden. It is not the area, which measures the space inside the circle and would be given in square units, such as $\text{m}^2$.