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Area of a Circle

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Area of a Circle Formula

How to Find the Area of a Circle

Where does the formula come from?

Archimedes derived the formula for the area of a circle by viewing it as the area of a regular polygon with infinitely many sides.

This activity is one of my favorite ways to visualize the formula for the area of a circle. 

You can print out this fraction circle worksheet if you want to try this activity by hand. Or you can check out this applet that allows you do to it virtually. 

Cut up a circle into 6, 8, 10, or 12 equal-sized pieces and then rearrange the pieces side by side like this...

When the pieces are arranged like this, the circumference of the circle is split into two equal lengths shown by the red, curvy lines.

Notice that the rearranged pieces create the rough shape of a parallelogram.


The height of the parallelogram is the radius of the circle (r).

The base of the parallelogram is approximately half of the circumference of the circle. It is not exact because the red lines are curved, but it is pretty close.

If you cut the circle up into smaller pieces, the red curvy line will look more and more like a straight line and the parallelogram will look more and more like a rectangle. 

This applet will let you cut up the circle into 200 pieces, if you want to try it.

The formula for the circumference of the circle is \(2\pi r\). and half of that is \(\pi r\). This is the base of the parallelogram that is made from the cut-up pieces of the circle.

To find the area of the parallelogram, multiply the base of the parallelogram (\(\pi r\)) by the height (r).

The area of the parallelogram (\(\pi r\)\(r\) or \(\pi r^{2}\)) is the same area as the circle because the pieces of the circle made the parallelogram. So, that is why the formula for the area of a circle is \(\pi r^{2}\).

Area of a Circle from Diameter

Area of a Circle from Circumference

Area of a Sector

Radius, Diameter, Circumference from Area 

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