Circle Calculator
A circle is defined by a single measurement — its radius. From the radius, every other property follows exactly: the diameter is twice the radius, the circumference is 2πr, and the area is πr². This calculator lets you start from whichever measurement you know — radius, diameter, circumference, or area — and instantly computes the other three with full precision.
Whether you are working in geometry, engineering, architecture, or everyday projects, circle calculations appear constantly. Knowing the relationship between radius, diameter, circumference, and area allows you to switch between measurements without manual formula lookup. Enter any one value and all four properties are resolved in real time.
How it works
r = radius; d = 2r; C = 2πr; A = πr². Inverse derivations: from diameter r = d/2; from circumference r = C/(2π); from area r = √(A/π).
Use cases
- Calculating the amount of fencing needed to enclose a circular garden
- Finding the area of a circular floor for tiling or carpet estimates
- Converting between wheel diameter and circumference for speed calculations
- Determining pipe cross-section area in plumbing or engineering design
- Computing circular track lengths for athletics or motorsport planning