Circle Calculator
Calculate circle area, circumference, diameter, radius, arc length, and sector area from any single measurement.
Inputs
Results
Circle Formulas
Circumference: C = 2πr = πd. Area: A = πr². Arc length: L = (θ/360) × 2πr. Sector area: A = (θ/360) × πr². A circle's area is exactly π times the square of its radius — π ≈ 3.14159265...
What is the Circle Calculator?
The Circle Calculator is a specialized geometry tool. If you only know one single metric about a circle (like its radius or circumference), this calculator will instantly reverse-engineer and compute all the remaining missing properties.
How It Works (Formula)
Every property of a circle is mathematically locked to the irrational number Pi ($\pi$). The Area is $\pi$ multiplied by the radius squared. The Circumference (perimeter) is $\pi$ multiplied by the diameter. Knowing just one unlocks the rest.
$$ A = \pi r^2 \quad \text{and} \quad C = 2\pi r $$
The fundamental geometry equations for circles.
How to Use It
You only need to enter one measurement. Provide either the Radius, Diameter, Circumference, or Area. The calculator will run the algebraic algorithms backward and forward to fill out the remaining three properties.