What do a pizza, donuts, a truck tire, and a Frisbee have in common?

They are all circular. There are lots of circles all around us.

Circumference is the term for the perimeter of a circle — the distance around the outside of the circle. Before you can begin to find the circumference of a circle, you must know two other terms.

The diameter of a circle is a straight line going from one side of the circle to the other side of the circle passing through the centre of the circle.

The radius of a circle is a straight line from one side of the circle to the centre of the circle. It is half the length of the diameter of the circle.

You can estimate the circumference of a circle by following this simple method:

1. Place a piece of string around the edge of a circle.
2. Cut the string and lay it along the side of a ruler to find the circumference.

You can also estimate the diameter of a circle by using a measuring tape, a ruler, or a metre stick.

Try This:

Find two circular objects with different diameters. You might use a juice can, a soup can, a coffee can, or even a garbage can lid.

Use the string and ruler/metre stick or a measuring tape to measure the circumference of the tops of the objects. Then measure the length of the diameter.

Complete a chart like this:

How does the measurement of the circumference compare to the measurement of the diameter? Is it twice as large? Is it three times as large or more than three times as large?

This comparison is the ratio of the circumference to the diameter of the circle. This ratio is called pi and the symbol for pi is π. You will learn more about pi below.

In the column marked COMPARISON, list the answer for the circumference divided by the diameter. Show your teacher your completed chart.

Early mathematicians who studied circles discovered that the circumference of circles divided by the diameter equals approximately 3.14.

The value 3.14 is called pi, which is a mathematical constant common to all circles, no matter how big or small.

The circumference of a circle can be calculated using two different formulas.

If you know the diameter, use this formula:

C = πd (Circumference = pi x diameter)

If you know the radius, use the formula:

C = 2πr (Circumference equals pi times two times the radius)

Here are some examples:

Try This:

You have a truck tire which is 40 cm in diameter. Use the correct formula from above to find the circumference of this tire.

You have a bicycle tire with a radius of 9 cm. Use the correct formula from above to find the circumference of this tire.

You may use a calculator to solve.