Circle Area Calculator: Quick and Easy Results

What it computes

Enter either the radius or the diameter (the tool halves the diameter internally if that's what you pick) and it returns all three related measurements at once: area (πr²), circumference (2πr), and diameter (2r). π is taken from JavaScript's built-in Math.PI, accurate to about 15 significant digits — far more precision than the 4 decimal places shown in the result.

Worked example

A radius of 5 cm gives: Area = π × 5² = π × 25 = 78.5398 cm². Circumference = 2 × π × 5 = 31.4159 cm. Diameter = 2 × 5 = 10 cm. If you'd entered 10 cm as the diameter instead, the tool would divide by 2 to get the same radius of 5 and produce identical results.

Why area and circumference use different powers of r

Circumference scales linearly with radius (double the radius, double the perimeter) because it's a 1-dimensional measurement of the boundary. Area scales with r² because it's a 2-dimensional measurement of the enclosed surface — doubling the radius quadruples the area. This is why a radius-10 circle has 4× the area but only 2× the circumference of a radius-5 circle.

Constraints

Zero and negative values are rejected — a circle needs a positive radius. The unit dropdown (cm, m, in, ft) is a label only; it does not convert between units, so results stay in whatever unit you selected for input.