Circle Calculator

Enter one measurement

How it works

Four formulas tie everything together: d = 2r, C = 2πr, and A = πr². Given the circumference, the radius is recovered as r = C ÷ 2π; given the area, r = √(A ÷ π). Once the radius is known, the rest follow directly. Units carry through: enter inches and the area is square inches.

About π precision: π is irrational — it never ends or repeats. This page uses JavaScript's built-in Math.PI, which carries π to about 15–16 significant digits (3.141592653589793…), far more than any tape measure, lathe, or pizza can use. Displayed results are rounded to 4 decimal places by default; the underlying math is not rounded at any step.

Worked example: a radius of 5 gives diameter 10, circumference 31.4159, and area 78.5398. Entered the other way — an area of 78.5398 — you get the same radius of 5 back.

Frequently asked questions

How do I find the area from the diameter? Halve the diameter to get the radius, then square it and multiply by π. A 10-inch pizza (diameter 10, radius 5) has an area of about 78.54 square inches.

Is circumference the same as perimeter? Yes — circumference is the name for a circle's perimeter, the distance once around the edge.

Why does the area grow so fast? Because the radius is squared in A = πr². Double the radius and the area quadruples — a 20-inch pizza is four times the food of a 10-inch one, not two times.