Calculate the area of a circle from radius or diameter with circumference and diameter included.
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Also in Circles
Geometry
Calculate the area of a circle by entering the radius or diameter. The calculator also shows the circumference and diameter for a complete picture of the circle.
Area = pi times radius squared. If you know the diameter instead, divide it by 2 to get the radius first, then apply the formula.
Pi (approximately 3.14159) is the ratio of a circle's circumference to its diameter and appears in all circle calculations.
A = pi * r^2
Area from radius.
This calculator assumes a perfect circle. For ellipses or irregular shapes, use a dedicated tool.
Frequently asked questions
Divide the diameter by 2 to get the radius, then use A = pi * r^2. The calculator does this automatically in diameter mode.
Square units matching your input — if the radius is in centimetres, the area is in square centimetres.
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