Area of a Circle

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Area of a Circle Part I

Unit 2 > Lesson 3 of 6

The distance around a circle is called its circumference. The distance across a circle through its center is called its diameter. We use the Greek letter

(pronounced Pi) to represent the ratio of the circumference of a circle to the diameter. In the last lesson, we learned that the formula for circumference of a circle is:

. For simplicity, we use

= 3.14. We know from the last lesson that the diameter of a circle is twice as long as the radius. This relationship is expressed in the following formula:

	The area of a circle is the number of square units inside that circle. If each square in the circle to the left has an area of 1 cm², you could count the total number of squares to get the area of this circle. Thus, if there were a total of 28.26 squares, the area of this circle would be 28.26 cm² However, it is easier to use one of the following formulas:
	or
where is the area, and is the radius. Let's look at some examples involving the area of a circle. In each of the three examples below, we will use = 3.14 in our calculations.