Circle Circumference Calculator

C = 2πr (or πd). Area = πr². The fundamental circle formulas.

Inputs

Result

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How to use this calculator

  • Pick radius or diameter.
  • Enter value.
  • Read circumference + area.

About this calculator

A circle's circumference C = 2πr = πd. π ≈ 3.14159 — the ratio C/d is constant for every circle. Area scales as πr² — doubling radius quadruples area. Used in everything from wheels to pizza to satellite orbits. The number π itself was known to Babylonian and Egyptian mathematicians ~2000 BCE; Archimedes proved 3.140845 < π < 3.142857 around 250 BCE.

Frequently asked

Why π?+
It's the ratio of circumference to diameter — constant for any circle. Defined as C/d.
How precise is π in JavaScript?+
Math.PI = 3.141592653589793 (15 sig figs). For ultra-precise calculations, use a big-decimal library.
Diameter or radius?+
Diameter = 2 × radius. Both common; pick whichever the problem gives you.
Real-world use?+
Wheel rotation (C × revolutions = distance). Pipe / wire / cable wrapping. Track running (lap distance).
Circle vs. ellipse?+
Circle is a special ellipse (semi-axes equal). Ellipse perimeter has no closed-form — see Ramanujan approximation.

Related calculators

Ellipse Perimeter Calculator (Ramanujan)
P ≈ π(a + b)(1 + 3h / (10 + √(4 − 3h))) where h = ((a−b)/(a+b))². Ramanujan approximation.
Cylinder Volume Calculator
V = πr²h. Surface area = 2πr² + 2πrh (top + bottom + side).
Cone Volume Calculator
V = (1/3)πr²h. One-third of the cylinder of same base and height.
Sphere Volume Calculator
V = (4/3)πr³. Surface area = 4πr².
Triangle Perimeter Calculator
P = a + b + c. Sum of all three sides.
Rectangle Perimeter Calculator
P = 2(l + w). Length plus width, doubled.

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