Skill: Calculate the circumference of circles

The circumference of a circle is the distance around it.

Use one of these formulas:

• If you know the diameter $d$:

$$C=\pi d$$

• If you know the radius $r$:

$$C=2\pi r$$

Remember that the diameter is twice the radius: $(d=2r)$. The answer must be in linear units, such as centimetres or metres.

Example

A circle has a radius of $(6\text{ cm})$. Find its circumference.

Step 1: Choose the formula.

The radius is given, so use

$$C=2\pi r$$

Step 2: Substitute the radius.

$$C=2\pi(6)$$

Step 3: Simplify.

$$C=12\pi\text{ cm}$$

This is the exact circumference. Using $\pi\approx3.14$,

$$C\approx12(3.14)=37.68\text{ cm}$$

So the circumference is

$$\boxed{12\pi\text{ cm}\approx37.7\text{ cm}}$$

Use $\pi$ when an exact answer is requested, and use a decimal approximation when a rounded answer is requested.