Circle Calculator
Find a circle’s radius, diameter, circumference and area from any one.
Results use a consistent unit. Area is in square units of whatever length unit you enter.
Everything from one measurement
A circle is defined by a single number — its radius. Once you know that, every other property follows from π, the constant linking a circle’s size to its perimeter and area.
d = 2r · C = 2πr · A = πr²
Enter whichever measurement you have and the calculator works back to the radius first, then forward to the rest. That means you can start from a known circumference or area just as easily as from the radius.
A circle with a radius of 5 has a diameter of 10, a circumference of about 31.42 and an area of about 78.54 — that is 2πr for the perimeter and πr² for the area.
Where circles turn up
From pizza sizes and pipe diameters to garden ponds and running tracks, circle maths is everywhere. The area formula in particular surprises people: doubling the radius quadruples the area, since it depends on the radius squared.
Worth remembering
- Area scales with the square. A 16-inch pizza has far more than twice the food of an 8-inch one.
- Radius is half the diameter. A common slip when reading off measurements.
- Keep units consistent. Length stays linear; area becomes square units.