About the circle calculator
Every circle is determined by one number. The radius r is the distance from the center to the edge, the diameter is twice the radius, the circumference is the distance around (2πr) and the area is πr².
Results are also shown as exact multiples of π where possible, a circle of radius 5 has area exactly 25π.
Formulas
| Area | A = πr² |
| Circumference | C = 2πr = πd |
| Diameter | d = 2r |
| Radius from area | r = √(A/π) |
Frequently asked questions
How do I find the radius from the circumference?
Divide by 2π: r = C/(2π). A circumference of 31.4159 gives r ≈ 5.
How do I find the area from the diameter?
A = π(d/2)² = πd²/4. A 10 cm diameter gives 25π ≈ 78.54 cm².
What is π?
π ≈ 3.14159265 is the ratio of any circle’s circumference to its diameter. The calculator uses full double-precision π.