Circle Area & Circumference Calculator
Circle calculator: area from radius (A = πr²) or radius from area, plus diameter and circumference in any length unit.
A = π · r²
How to use this calculator
- Choose the quantity you want under “Solve for”.
- Type the known values and pick their units: any combination works.
- The result updates as you type. Open “Show the steps” to see the substitution.
The area of a circle is A = π·r² and its circumference is C = 2·π·r = π·d, with π ≈ 3.14159265.
Solve for the radius from a known area, for example to size a round pipe or tank. Area units adapt to the length unit you choose.
Formula
A = π · r²
| Symbol | Quantity | Units |
|---|---|---|
| r | Radius | m, km, cm, mm, µm, nm, pm, Å, in, ft, yd, mi, nmi, au, ly |
| A | Area | m², km², cm², mm², ha, ft², in², yd², ac, mi² |
Worked example
Given: r = 5 cm
A = π · r²
Result: A = 78.5398 cm²
Frequently asked questions
What is the area of a circle with a 10 cm diameter?
r = 5 cm, so A = π · 25 = 78.54 cm².
How do I get the radius from the circumference?
r = C/(2π).