How to Use the Circle Area Calculator
This calculator finds all circle measurements from any one of three inputs: radius, diameter, or circumference. It's ideal for geometry problems, DIY projects, and engineering calculations.
Circle Formulas
From radius r: Diameter d = 2r, Circumference C = 2πr, Area A = πr². π ≈ 3.14159265. This calculator uses JavaScript's Math.PI for maximum precision.
Reverse Calculations
From diameter: r = d ÷ 2. From circumference: r = C ÷ (2π). All three input modes are supported — just select the one you know and the calculator handles the rest automatically.
Units
Results use the same unit as your input. If the radius is in inches, the circumference is in inches and the area is in square inches. Make sure all your measurements share the same unit before entering.
Frequently Asked Questions
Pi is the ratio of a circle's circumference to its diameter. It's approximately 3.14159265 and is irrational (it never repeats or terminates). This calculator uses the full precision available in JavaScript.
Area is in (length unit)². If you enter the radius in meters, the area is in m². Enter in centimeters and the area is in cm². No automatic unit conversion is done.
Area = π × 5² = 25π ≈ 78.5398. Enter 5 as the radius in this calculator to verify instantly.