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Circle Calculator
Enter the radius — area and circumference update instantly.
Formula
Area = πr² | Circumference = 2πr = πd | d = 2r
Example
r = 50 mm → Area = 7854 mm² ≈ 78.5 cm², circumference = 314.16 mm.
FAQ
How do I calculate the area of a circle?
Square the radius and multiply by π: Area = πr². For diameter d it is πd²/4.
What is π exactly?
π ≈ 3.14159 is the ratio of circumference to diameter. This calculator uses full double-precision π.