Every property of a circle flows from one number: the radius. Give this calculator the radius and it returns the three quantities you actually need — area, circumference, and diameter — with π handled to full precision.
How it works
The area grows with the square of the radius (double the radius, quadruple the area), while circumference and diameter grow linearly. The calculator applies all three formulas at once.
The formula
r is the radius; π ≈ 3.14159. If you know the diameter instead, halve it first; if you know the circumference, r = C ÷ 2π.
Worked example
A circle of radius 5: area = π × 25 ≈ 78.54, circumference = 10π ≈ 31.42, diameter = 10 — all in the square and linear units of whatever you measured the radius in.
Frequently asked questions
How do I get the radius from an area?
Invert the formula: r = √(A ÷ π). A pizza advertised as 1,000 cm² has radius √(1000 ÷ π) ≈ 17.8 cm — about a 36 cm pizza.
Why is a 40 cm pizza much more than double a 20 cm one?
Because area scales with the radius squared: doubling the diameter quadruples the area. The 40 cm pizza has four times the food — the single most useful circle fact for everyday value comparisons.
What exactly is π?
The ratio of any circle's circumference to its diameter — the same irrational constant (3.14159…) for every circle in the universe, which is why it appears in both formulas.