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Circle Calculator Guide
Calculate every measurement of a circle from any one value. Enter the radius, diameter, circumference, or area, and the calculator finds the other three.
Diameter = 2r · Circumference = 2πr · Area = πr². From area, radius = √(area ÷ π); from circumference, radius = circumference ÷ 2π.
How Do I Find the Area of a Circle?
Multiply pi (about 3.14159) by the radius squared. For example, a circle with a radius of 5 has an area of π × 25 ≈ 78.54 and a circumference of 2π × 5 ≈ 31.42.
Radius, Diameter, and Pi
The radius is the distance from the center to the edge, and the diameter is twice that, straight across. Pi is the ratio of any circle's circumference to its diameter, roughly 3.14159 for every circle.
Related: Area Calculator, Volume Calculator, and Square Footage.
Frequently Asked Questions
How do I calculate the area of a circle?
Multiply pi by the radius squared (πr²); a radius of 5 gives an area of about 78.54.
How do I find the circumference?
Multiply 2 by pi by the radius (2πr), or pi by the diameter.
What is the radius if I know the area?
Divide the area by pi and take the square root: radius = √(area ÷ π).
What is the diameter?
The diameter is twice the radius — the distance straight across the circle through the center.
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