Circle Calculator
Enter any one known value — radius, diameter, circumference, or area — to calculate the rest.
Known value
Radius:
Diameter:
Circumference:
Area:
The relationships between radius, diameter, circumference, and area
All four circle measurements are mathematically linked through π: diameter = 2 × radius, circumference = 2πr (or πd), and area = πr². Because they're all derived from the same radius, knowing any single one lets you solve for the other three — which is exactly what this calculator does regardless of which value you start with.
Frequently asked questions
Why does area use radius squared but circumference doesn't? Circumference is a one-dimensional length that scales linearly with radius, while area is a two-dimensional measurement that scales with the square of radius — doubling the radius doubles the circumference but quadruples the area.
What is π, exactly? Pi (π ≈ 3.14159) is the constant ratio of any circle's circumference to its diameter — the same value for every circle regardless of size, which is why it appears in all circle formulas.
Can I use this for a sphere's surface area or volume? No — this calculator covers flat, two-dimensional circle measurements only. Sphere formulas (4πr² for surface area, (4/3)πr³ for volume) use radius differently and aren't included here.