How it works
A circle is defined by a single measurement. From the radius r, the diameter is 2r, the circumference is 2πr, and the area is πr².
Choose which value you know. If you know the area, the radius is √(A/π); if you know the circumference, the radius is C/2π. Results use the full precision of π (3.14159265…).
Formula
d = 2r; C = 2πr = πd; A = πr²
Questions people ask
What is the area of a circle with a 10-inch diameter?
The radius is 5 in, so the area is π × 5² ≈ 78.54 square inches.
How do I find the radius from the circumference?
Divide by 2π. A circumference of 31.4 gives a radius of about 5.
Is a 16-inch pizza twice as big as a 12-inch?
Not quite: area grows with the square of the diameter. A 16-inch pizza is (16/12)² ≈ 1.78 times the area of a 12-inch one.