How to calculate circle
Area = πr² · Circumference = 2πr · Diameter = 2r
Worked example. Area: 78.5398 square units.
Example inputs — Radius: 5. Starting values are illustrative.
From George Guju, founder of Calculyn · Educational calculator guidance, not professional advice.
How it works
Every circle measurement comes from the radius and π (about 3.14159). Area grows with the square of the radius.
Assumptions & limitations
- Results are rounded for display.
Formula and assumptions reviewed October 2026 by the Calculyn team. How we build our calculators
Frequently asked questions
I only know the diameter.
Divide it by 2 to get the radius, then enter that.