Circle Calculator
Variable Key
Given radius r
Calculate diameter, circumference, and area from the radius.
Given diameter d
Calculate radius, circumference, and area from the diameter.
Given circumference C
Calculate radius, diameter, and area from the circumference.
Given area A
Calculate radius, diameter, and circumference from the area.
A circle is the set of all points in a plane that are exactly the same distance (the radius) from a central point. All four measurements — radius, diameter, circumference, and area — can be calculated from just one known value using the mathematical constant π (pi ≈ 3.14159).
Tip: The circumference of the Earth at the equator is about 40,075 km. Its radius is about 6,371 km. Check: 2π × 6,371 ≈ 40,030 km — close!
Fun Fact
A circle has the largest area of any shape with the same perimeter — this is called the isoperimetric inequality. Nature uses this: soap bubbles and planets form spheres for the same reason.
References