Circle Calculator

From the radius r: diameter = 2r, circumference = 2 x pi x r, area = pi x r^2. Working backwards, r = circumference / (2 x pi), and r = the square root of (area / pi).

What is the difference between radius and diameter?

The radius runs from the center to the edge and the diameter runs all the way across through the center. The diameter is exactly twice the radius, and mixing them up is the single most common mistake in circle math.

How do I find the area of a circle from its circumference?

Divide the circumference by 2 pi to get the radius, then square it and multiply by pi. This calculator does that if you pick circumference in the dropdown.

How do I measure a circle I cannot reach the center of?

Measure around it with a tape, which gives the circumference, then work back. That is usually far easier in practice than trying to find the exact center of a tree trunk or a pond.