Circle Calculator
Enter any one of radius, diameter, circumference or area and the calculator returns the other three, along with the exact formulas used. Rounding is adjustable and every value is computed at full double precision internally.
Enter one value
Enter a value and press Calculate.
Circle formulas
Diameter: d = 2r
Circumference: C = 2πr = πd
Area: A = πr² = πd²/4
Where π ≈ 3.141592653589793. Every value can be derived from any one of the four, so choosing one input uniquely determines the rest.
Worked examples
Radius → everything
r = 5 → d = 10, C = 10π ≈ 31.4159, A = 25π ≈ 78.5398
d = 10, C ≈ 31.4159, A ≈ 78.5398
Area → radius
A = 50 → r = √(50/π) ≈ 3.9894, d ≈ 7.9788, C ≈ 25.0663
r ≈ 3.9894
Related calculators
Frequently asked questions
The calculator uses JavaScript's Math.PI, accurate to about 15 significant digits — the limit of IEEE-754 double precision.
Yes — select "Circumference" from the dropdown and enter the value. The calculator will derive radius, diameter and area.