Every circle measure from any one
A circle is fixed entirely by its radius — give any single measurement and the rest follow. Tell this calculator whether you know the radius, diameter, circumference, or area, enter the value, and it returns all four at once.
The formulas
With radius r: diameter = 2r, circumference = 2 pi r, and area = pi r squared. Working backward, radius = diameter / 2, or circumference / (2 pi), or the square root of (area / pi). The constant pi is about 3.14159.
Radius is the key
Whatever you start from, the tool first recovers the radius, then builds the other three from it. That is why a single number is enough to pin down the whole circle.
Related geometry tools
For the 3D version, use the sphere calculator; for a circle extruded into a tube, the cylinder calculator.
Worked example
A radius of 5 gives a diameter of 10, a circumference of about 31.42, and an area of about 78.54 — since area is pi times 25.
FAQ
Why does area grow so fast?
Area depends on the radius squared, so doubling the radius quadruples the area. Circumference, by contrast, only doubles.
What units does it use?
Whatever unit you enter — the outputs share it, with area in those units squared. The math is unit-agnostic.