Circle Calculator
Calculate radius, diameter, circumference, and area of a circle from any known value.
A circle calculator finds the radius, diameter, circumference, and area of a circle from any one known measurement.
Examples
Radius = 5
Diameter = 14
From area
Frequently Asked Questions
What is pi (π)?
How do I find the radius from the area?
What is the relationship between radius and diameter?
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.
A circle calculator finds the radius, diameter, circumference, and area of a circle from any one known measurement.
How to Use This Calculator
Select which property you know (radius, diameter, circumference, or area) and enter its value. The calculator will compute all other circle properties.
Understanding the Formula
Diameter = 2r; Circumference = 2πr; Area = πr²; where r is the radius and π ≈ 3.14159.
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.
Assumptions & Limitations
- Assumes Euclidean geometry.
- Assumes exact input values; rounding in inputs propagates to results.
- Results may show floating-point approximations for irrational numbers.