Circle Calculator
Enter any one of radius, diameter, circumference, or area, and instantly get the other three. Solves a complete circle from a single value.
Radius
5
Diameter
10
Circumference
31.415927
Area
78.539816
d = 2r · C = 2πr = πd · A = πr². Enter any one value and the calculator solves for the radius first, then the remaining three. Area is in square units; the others share the unit you entered.
Frequently Asked Questions
How do I find the circumference of a circle?
Multiply the diameter by pi, or the radius by 2π: C = πd = 2πr. For a circle with radius 5, the circumference is 2 × π × 5 ≈ 31.42. Enter the radius above and the calculator returns the circumference instantly.
How do I find the radius from the circumference?
Divide the circumference by 2π: r = C ÷ (2π). For a circumference of 31.42, the radius is 31.42 ÷ 6.283 ≈ 5. Select 'Circumference' as the known value and the calculator works backward to the radius.
How do I find the area of a circle from the diameter?
First halve the diameter to get the radius (r = d ÷ 2), then use A = πr². The calculator handles this automatically — choose 'Diameter' as your known value and it will report the area along with the radius and circumference.
What is the relationship between radius and diameter?
The diameter is exactly twice the radius (d = 2r), and the radius is half the diameter (r = d ÷ 2). The radius runs from the center to the edge; the diameter runs all the way across through the center.
Can I solve a circle from just one measurement?
Yes. Given any single value — radius, diameter, circumference, or area — every other property of the circle is determined. This calculator solves for the radius first and then computes the other three.