What's the formula for the area of a circle?

Area = π × r², where r is the radius. With r = 5, the area is π × 25 ≈ 78.54 square units. If you know the diameter d, use Area = π × (d/2)².

How are area and circumference related?

Both depend on the radius and π. Circumference = 2 × π × r is linear in r, while area = π × r² grows with the square of r. So doubling the radius doubles the circumference but quadruples the area.

π (pi) is the ratio of any circle's circumference to its diameter — the same constant for every circle, ≈ 3.14159. It's irrational and shows up in every circle, sphere, and wave equation.

Why are radius and diameter listed separately if one is just twice the other?

Convenience. Real-world measurements often give you whichever is easier to measure — a pipe's diameter, a wheel's radius — and seeing both at once means you don't have to do the doubling or halving in your head.

GEOGeometry

Circle Calculator

Enter any one of radius, diameter, circumference, or area, and get the other three instantly. Switch between meters, centimeters, feet, and inches — all four results share the same unit so you can compare directly.

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How It Works

Formula

A = \pi r^2

C = 2 \pi r

d = 2\,r

r = \sqrt{A / \pi}

Where

$r$: Radius (distance from center to edge)
$d$: Diameter (full width through the center)
$C$: Circumference (perimeter of the circle)
$A$: Area (region enclosed by the circle)
$\pi$: Pi — the ratio of circumference to diameter, ≈ 3.14159

Pick what you have, enter the value. The calculator solves for the radius first, then derives the other three using π. All outputs are shown in the unit you selected — area in unit².

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