How to Use the Area of a Circle Calculator

Choose Radius or Diameter, enter the value, and the area fills in below.

area = π × radius²
area = π × (diameter ÷ 2)²

Squaring the radius is why area grows so much faster than circumference as a circle gets bigger — double the radius and the circumference only doubles, but the area quadruples, which is the same reason a 16-inch pizza is a lot more than twice an 8-inch one despite looking only "twice as big" across.