Circle Area Calculator
Find the area inside a circle from its radius. The result uses square units, such as cm², and the radius is half the diameter.
How to use Circle Area Calculator
1. Choose Measurement Unit, then enter Radius without a unit suffix. 2. Use the selected ordinary, square or cubic unit for each measurement as explained below; changing the selector relabels inputs and does not convert them. 3. Select Calculate, or press Enter, and read circle area. 4. Select Select Copy result to copy the answer, or Reset to clear the form.
Why use this tool?
Estimate the flat area of a round tabletop, label or circular garden bed before allowing for waste or material thickness.
How the calculation works
Area = π × radius². The tool uses the browser’s value of π rather than a rounded 3.14. Results use up to six decimal places. Tiny nonzero results below 0.000001 use scientific notation with six significant digits. Browser floating-point arithmetic can introduce small rounding differences. Inputs and calculated magnitudes must not exceed 9,007,199,254,740,991. Negative physical measurements and arithmetic underflow are rejected. Zero dimensions are treated as a degenerate limit where the formula allows them; divisors and sides of an actual triangle must be positive. Formula reference: https://openstax.org/books/prealgebra-2e/pages/c-geometric-formulas
Example
A radius of 5 cm gives π × 25 = 78.539816 cm².
Practical tip
If you measured across the whole circle, divide that diameter by two before entering Radius.
Frequently asked questions
What happens if I enter the diameter as the radius?
The calculated area becomes four times too large, because the radius is squared. A 10 cm diameter has a 5 cm radius.
Does the result include a border or thickness?
It is the flat area inside the circle. A border, curved surface or material thickness needs a separate calculation.
Does changing Measurement Unit convert my numbers?
No. The selector states the unit of the numbers you enter and labels the answer. Convert measurements first when changing units. Area inputs use square units and volume inputs use cubic units; result precision follows the rounding method above.