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Circle Calculator
Find the area, circumference, radius, or diameter of a circle when you know any one of them.
Greater than zero
Labels the results, such as cm or in. Area is shown with that unit squared. Nothing is converted.
Formula
d = 2r
C = 2πr
A = πr²
Enter a radius greater than zero to see the diameter, circumference, and area.
This circle calculator finds area, circumference, radius, and diameter when you know any one of them. A radius uses A = πr², C = 2πr, and d = 2r. A diameter, circumference, or area is turned back into the radius first (d/2, C/(2π), or √(A/π)), and the other two measurements follow. You can type a unit such as cm or in to label the results; the tool does not convert units.
One measurement determines the area, circumference, radius, and diameter.
The area of a circle is A = πr², the circumference is C = 2πr, and the diameter is d = 2r. A radius of 7 has diameter 14, circumference 14π, and area 49π.
A diameter uses r = d/2, C = πd, and A = πd²/4. Circumference gives r = C/(2π), and area gives r = √(A/π). The calculator takes one of those four numbers and fills in the other three.
Frequently Asked Questions
What is the area of a circle formula?
The area is π times the radius squared: A = πr². A radius of 7 has area 49π, about 153.938. If you know the diameter instead, use A = πd²/4.
What is the formula for the circumference of a circle?
Circumference is the distance around the circle: C = 2πr, which is the same as π times the diameter. A radius of 7 has circumference 14π, about 43.982. From a diameter, multiply that diameter by π.
How do you find the radius of a circle from the area?
Divide the area by π and take the square root: r = √(A/π). The diameter is twice that radius, and the circumference is 2πr.
How do you find the radius from the diameter?
The radius is half the diameter: r = d/2. A diameter of 14 has radius 7, circumference 14π, and area 49π.