Find the area, circumference, and diameter of a circle from its radius. Enter the radius.
Usage Tip
Double the radius and the circumference doubles, but the area quadruples — area grows with the square.
circumference = 2 × π × radius
Area scales with the square of the radius; circumference scales linearly.
Results use pi to full precision.
Area is in square units, circumference in linear units.
A circle is defined by its radius, the distance from the center to any point on the edge. The diameter is twice the radius, the circumference is pi times the diameter, or two times pi times the radius, and the area is pi times the radius squared. Pi is about 3.14159, the ratio of any circle’s circumference to its diameter.
Because area depends on the radius squared, doubling the radius makes the area four times larger. A common mistake is to enter the diameter where the radius is expected, which makes the area four times too big. If you only know the circumference or the area, you can work backward to the radius with the same formulas. Circles come up in pipes, wheels, tanks, round tables, and any layout with a circular shape.