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Area enclosed by a circle
From Latin: area - "level ground, an open space,"
The number of square units it takes to exactly fill the interior of a circle.
Try this Drag the orange dots to move and resize the circle. As the size of the circle changes, the area is recalculated.
(If there is no image below, see support page.)
A circle is actually a line, one that connects back to itself making a loop. Imagine the circle to be a loop of string. The string itself has no area, but the space inside the loop does. So strictly speaking a circle has no area.

However, when we say "the area of a circle" we really mean the area of the space inside the circle. If you were to cut a circular disk from a sheet of paper, the disk would have an area, and that it what we mean here.


If you know the radius
Given the radius of a circle, the area inside it can be calculated using the formula
where:
R  is the radius of the circle
π  is Pi, approximately 3.142


If you know the diameter
If you know the diameter of a circle, the area inside it can be found using the formula
where:
D  is the diameter of the circle
π  is Pi, approximately 3.142


If you know the circumference
If you know the circumference of a circle, the area inside it can be found using the formula
where:
C  is the circumference of the circle
π  is Pi, approximately 3.142
Try this
  1. In the figure above, click on "hide details"
  2. Drag the orange dot on the edge of the circle to make a random-size circle.
  3. Now try to estimate the area enclosed by the circle just looking at the squares inside it
When you done click "show details" to see how close you got.

Other circle topics

General

Angles in a circle

Arcs

(C) 2007 Copyright John Page