Circle Calculator
Enter any one measurement of a circle and get all the others.
The circle formulas
C = 2πr A = πr²
Everything about a circle follows from the radius. Given the diameter, halve it; given the circumference, divide by 2π; given the area, divide by π and take the square root. A radius of 5 gives diameter 10, circumference 31.42 and area 78.54.
A surprising consequence
Because circumference grows linearly with radius, adding 1 meter of rope around ANY circle raises it about 16 cm off the surface, whether the circle is a coin or the equator. Area is the one that grows with the square: double the radius, quadruple the pizza.