Area of a Circle Calculator
Find the area of a circle from its radius using A = π × r². The area is the flat space enclosed by the circle, measured in square units. Enter the radius and the calculator returns the area, scaling with the square of the radius — so doubling the radius quadruples the area.
Formula
Area = π × r²
- The area of a circle is π times the radius squared, where π ≈ 3.14159.
- Area scales with the square of the radius: doubling the radius multiplies the area by four.
- If you know the diameter d instead, halve it first (r = d ÷ 2), so the area becomes ¼ × π × d².
- The area is in the square of the radius's unit — a radius in cm gives an area in cm².
Example: circle of radius 5
Inputs
- Radius (r): 5
With r = 5, the area is π × 5² = 25π ≈ 78.54 square units, the circumference is 2π × 5 ≈ 31.42, and the diameter is 10.
Frequently asked questions
What is the formula for the area of a circle?
A = π × r², where r is the radius. Multiply the radius by itself, then by π (about 3.14159).
How do I find the area from the diameter?
Divide the diameter by 2 to get the radius, then apply A = πr². Equivalently, the area equals ¼ × π × d².
Why does the area grow so fast with radius?
Because the radius is squared. A circle with twice the radius has four times the area, and three times the radius gives nine times the area.
What units does the area use?
The area is in the square of whatever length unit you use. A radius in metres gives an area in square metres.
How do I get the radius back from the area?
Rearrange A = πr² to r = √(A ÷ π).