Circle Calculator

Calculate radius, diameter, circumference, and area of a circle from any known value.

Result

Fill in the fields above, then press Calculate.

A circle calculator finds the radius, diameter, circumference, and area of a circle from any one known measurement.

Select which property you know (radius, diameter, circumference, or area) and enter its value. The calculator will compute all other circle properties.

Examples

Radius = 5

Diameter = 10, Circumference = 31.416, Area = 78.540

Diameter = 14

Radius = 7, Circumference = 43.982, Area = 153.938

From area

If Area = 100, then r = √(100/π) ≈ 5.642

Frequently Asked Questions

What is pi (π)?
Pi is the ratio of a circle's circumference to its diameter, approximately 3.14159265359. It is an irrational number.
How do I find the radius from the area?
Use r = √(A/π). For example, if area = 50, then r = √(50/π) ≈ 3.989.
What is the relationship between radius and diameter?
The diameter is always exactly twice the radius: d = 2r.
Related Calculators
Area Calculator
Volume Calculator
Pythagorean Theorem Calculator
Triangle Calculator

Quick Tips

  • •Double-check your inputs — small errors lead to incorrect results.
  • •Use decimal form for fractions when entering values.
  • •Remember that area grows with the square of the radius, so doubling the radius quadruples the area.