Circle Calculator
Calculate all properties of a circle: radius, diameter, circumference, and area. Enter any one value to find the rest.
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Frequently Asked Questions
What is a circle?
A circle is a round plane figure whose boundary consists of points equidistant from a fixed center point. It's one of the fundamental shapes in geometry.
What is the formula for the circumference of a circle?
The circumference C = 2πr, where r is the radius. Alternatively, C = πd, where d is the diameter.
What is the formula for the area of a circle?
The area A = πr², where r is the radius. This represents the total space enclosed within the circle.
How do you find the radius from the circumference?
Divide the circumference by 2π: r = C / (2π). This reverses the circumference formula.
How do you find the radius from the area?
Take the square root of the area divided by π: r = √(A/π). This reverses the area formula.
What is the relationship between diameter and radius?
The diameter is exactly twice the radius: d = 2r. The diameter is the longest straight line that can be drawn through a circle, passing through the center.
What is π (pi)?
π is a mathematical constant approximately equal to 3.14159. It represents the ratio of a circle's circumference to its diameter and appears in all circle formulas.
Can you have a negative radius?
No, the radius must be a positive value. Negative values have no geometric meaning for a circle's dimensions.