Circle Calculator

Calculate circle properties: area, circumference, diameter, radius. Enter any one value, get all others. Uses constant π (pi) ≈ 3.14159265. Geometric relationships: diameter = 2 × radius; circumference = π × diameter = 2πr; area = πr².

Details

C (circumference) = 2πr = πd

A (area) = πr² = πd²/4

d (diameter) = 2r

r (radius) = d/2 = C/(2π) = √(A/π)

Tips

• Circumference: length around the circle
• Area: surface inside the circle
• Used for: round table size, pizza calculation, wheel design
• Sphere volume = (4/3)πr³ (different from circle)
• Annulus (donut shape): subtract inner circle from outer

FAQs

Why use π?

π is the ratio of circumference to diameter for ANY circle. Constant. Discovered by ancient Babylonians, refined over centuries.

Most decimal places of π?

Trillions known. For practical purposes, 10 digits (3.1415926536) is more than enough.

Circle vs sphere?

Circle is 2D (flat). Sphere is 3D (ball). Different formulas. Use sphere calculator for 3D objects.