CalcLibrary

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.

Examples

Classic textbook example — radius 5 m yields area ≈ 78.54 m² and circumference ≈ 31.42 m.

Area
78.54 m²
Circumference
31.42 m
Diameter
10.00 m
Radius
5.00 m

Solved from the radius — area scales with r², so a small change in r is amplified in the area.

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Examples

How It Works

Formula

A=πr2A = \pi r^2

C=2πrC = 2 \pi r

d=2 rd = 2\,r

d=C/πd = C / \pi

r=A/πr = \sqrt{A / \pi}

Variables

rr

Radius (distance from center to edge)

dd

Diameter (full width through the center)

CC

Circumference (perimeter of the circle)

AA

Area (region enclosed by the circle)

π\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. If circumference is the known value, diameter = circumference / π. All outputs are shown in the unit you selected — area in unit².

Frequently Asked Questions

01What'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.
02How 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.
03What is π?
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.
04Why 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.
05What if I only know the circumference?
That is enough. Diameter = circumference / π, and radius = circumference / (2π). From there, the area follows automatically.

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