Circle Calculator — Radius, Diameter, Circumference & Area
Enter any one circle measurement (radius, diameter, circumference, or area) to instantly calculate all the others. Full formulas and steps shown.
Enter any one value — all others will be calculated automatically.
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Radius
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Diameter
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Circumference
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Area
Formulas Used
Circle Formulas
Diameterd = 2r
CircumferenceC = 2πr = πd
AreaA = πr²
Radius from Cr = C / (2π)
Radius from Ar = √(A / π)
Frequently Asked Questions
What is the relationship between radius and diameter?
The diameter is exactly twice the radius: d = 2r. The radius is the distance from the center to any point on the circle; the diameter is the distance across the circle through its center.
How do I calculate the circumference of a circle?
Circumference C = 2πr = πd, where r is the radius and d is the diameter. Using π ≈ 3.14159, if a circle has radius 5, its circumference = 2 × π × 5 ≈ 31.416.
How do I find the area of a circle?
Area A = πr². If you know the diameter, A = π(d/2)². If you know the circumference C, then A = C²/(4π).
What is π (pi)?
Pi (π) is the ratio of a circle's circumference to its diameter. It is an irrational constant approximately equal to 3.14159265358979. It appears in formulas for circles, spheres, and many other geometric shapes.
Can I enter any of the four values to calculate the others?
Yes — this calculator accepts radius, diameter, circumference, or area as the starting input. Just type in any one field and all other values are calculated automatically.