Circle Calculator
Calculate area, circumference, diameter, and sector area from radius.
Formula
A = πr², C = 2πr, d = 2r
Area equals pi times radius squared. Circumference equals 2 times pi times radius.
Examples
Unit circle (r=1)
Circle with r=5
Circle with r=10
Frequently Asked Questions
How do I calculate the area of a circle?
Multiply π (≈3.14159) by the square of the radius: A = πr².
What is the circumference formula?
The circumference is C = 2πr, where r is the radius.
How is diameter related to radius?
The diameter is exactly twice the radius: d = 2r.
About this Calculator
Calculate area, circumference, diameter, and sector area from radius. All calculations are performed in your browser for instant results. Formulas are mathematically exact using JavaScript's built-in Math functions.