Circle Calculator
Calculate Area, Circumference, Diameter from Radius.
Input Values
Adjust parameters below
How It Works
Guide and Formula
Circle Geometry
A circle is a shape where all points are equidistant from the center.
Key Formulas
Area:
Circumference:
Diameter:
Where:
Relationship Between Measurements
Fun Fact
The ratio of circumference to diameter is always π, no matter the circle size!
?FAQs
What is the difference between radius and diameter?
Radius is the distance from the center to any point on the circle. Diameter is the distance across the circle passing through the center. Diameter = 2 × Radius.
Why is area πr² and circumference 2πr?
Area involves squaring the radius because it's a 2D measurement (covering a surface). Circumference is a 1D measurement (length of the boundary), so it uses just r.
What is pi exactly?
Pi (π) is the ratio of a circle's circumference to its diameter. It's an irrational number approximately equal to 3.14159, meaning its decimal representation never ends or repeats.
Can I calculate radius from area?
Yes! If you know the area A, then radius r = √(A/π). If you know circumference C, then radius r = C/(2π).