The area of a circle can be calculated using one of the following 3 formulas: one if you know the radius, another if you know the diameter and finally another, if you know the perimeter of a circle.

Formula Knowing the Radius

If you know the radius of the circle (the line from the center to the edge), you can calculate its area using the following formula:

$$"area" = pi * R^2$$

Circle Radius

Area Result

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If you know the diameter of the circle (the line that crosses the circle edge to edge through the center), you can calculate its area using the following formula:

$$"area" = (pi/4) * D^2$$

Circle Diameter

Area Result

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A circle is a shape or figure that is plane (2 dimensions) and perfectly round, with all points in the edge equidistant from the center.

The circle is a very basic geometric shape and a very common appearance in nature.

Circumference Definition

The circumference is the perimeter of a continuously curved shape, including a circle. It is the line around the curved shape that marks its boundary.

See perimeter.

Area Definition

It is the space of the internal surface in a figure, it is limited for the perimeter. Also, the area can be calculated in a plane of two dimensions.

Formula Definition

It is a representation of a rule or a general principle using letters. (Algebra, A. Baldor)
When describing formulas in plural, it is also valid to say "formulae".

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