Circle Area & Circumference Calculator
Find the area, circumference, and diameter of any circle from its radius using exact π and decimal formulas.
About the Circle Area & Circumference Calculator
A circle is the set of all points in a plane that are at a fixed distance—the radius (r)—from a center point. The distance across the circle through its center is the diameter (d = 2r), and the perimeter around the circle is the circumference (C).
Our Circle Calculator computes the exact area, circumference, and diameter of any circle both in terms of π and as rounded decimals.
Circle Geometry Formulas
Using the mathematical constant π ≈ 3.14159265 and radius r:
A = π × r²C = 2 × π × r = π × dd = 2 × rHow to Find the Area and Circumference of a Circle
- Find the Radius (r): If you are given the diameter, divide it by 2 to get the radius.
- Square the Radius for Area: Multiply
r × r, then multiply byπ ≈ 3.14159to get the area. - Double the Radius for Circumference: Multiply
2 × r × πto get the circumference.
Worked Example: For a circle with radius r = 7, the area is π × 7² = 49π ≈ 153.938, and the circumference is 2 × π × 7 = 14π ≈ 43.982.
Frequently Asked Questions (FAQ)
What happens to the area of a circle if you double the radius?
Because the radius is squared in A = πr², doubling the radius quadruples (multiplies by 4) the area of the circle.
Was this calculator helpful?
Help us improve our free step-by-step tools