Circle Calculator — Radius, Diameter, Circumference & Area
Know just one measurement of a circle? Pick which value you have, enter the number, and this tool solves for the radius, diameter, circumference, and area all at once.
📌 Enter a Known Value
📊 Result
Disclaimer: This tool performs standard mathematical calculations for educational and reference purposes. Always double-check results independently for graded coursework, exams, or safety-critical applications.
How Circle Measurements Relate to Each Other
A circle is one of the few shapes where a single measurement is enough to describe the whole thing. Because every point on the circle's edge sits the same distance from its center, the radius alone determines the diameter, the circumference, and the area — they're all locked together by the constant pi (π ≈ 3.14159).
The Formulas
Starting from the radius r: diameter d = 2r, circumference C = 2πr, and area A = πr². If you start from something other than the radius, this calculator works the formula backward: from diameter, r = d/2; from circumference, r = C / (2π); from area, r = √(A/π). Once r is recovered, every other property is computed the normal way.
Worked Example
Suppose a circular garden bed has a radius of 5 meters.
- Step 1: Diameter = 2 × 5 = 10 m
- Step 2: Circumference = 2 × π × 5 = 31.4159 m
- Step 3: Area = π × 5² = 78.5398 m²