Circle Calculator
Calculate the diameter, circumference, and area of a circle from its radius. Also supports reverse calculation: find the radius from circumference or area.
About this Calculator
Calculate the diameter, circumference, and area of a circle from its radius. Also supports reverse calculation: find the radius from circumference or area.
Formula & Calculations
Formula
d = 2r; C = 2πr; A = πr²; Reverse: r = C/(2π), r = √(A/π)Where:
- r=Radius of the circle
- d=Diameter (twice the radius)
- C=Circumference (distance around the circle)
- A=Area enclosed by the circle
Assumptions
- π is approximated as 3.1415926536 (Math.PI).
- All values use consistent units; input radius in a unit, output uses the same unit system.
- When calculating from circumference or area, the computed radius is used to derive all other values.
Calculation Examples
Example 1
Inputs:Radius = 5
Result:Diameter = 10, Circumference = 31.4159, Area = 78.5398
d = 2 × 5 = 10. C = 2π × 5 ≈ 31.4159. A = π × 25 ≈ 78.5398.
Example 2
Inputs:Area = 201.06 (find radius)
Result:Radius = 8, Diameter = 16, Circumference = 50.2655
r = √(201.06 / π) ≈ 8. From r, d = 16 and C ≈ 50.2655.
Frequently Asked Questions
Can I calculate the radius from the diameter?
Yes, simply set the diameter as the input value and the radius will be calculated as r = d/2, with all other values derived from that radius.
How accurate are the π calculations?
The calculator uses JavaScript's Math.PI constant, which provides approximately 15 decimal digits of precision — more than sufficient for any practical application.