How this calculation works
The radius runs from the centre to the edge. The diameter is twice the radius. Use one length unit consistently; the area is expressed in its squared form.
Area = πr²; circumference = 2πrA worked example
A radius of 5 units gives an area of approximately 78.539816 square units.
Good to know
Uses the stated formula and assumptions. Displayed decimal values are rounded; use consistent units for all inputs.
Twice the radius encloses four times the area
A circle with radius 10 has four times the area of a circle with radius 5, because radius is squared in πr². Its circumference is only twice as large because 2πr is linear in radius. Enlarging a shape changes lengths and areas at different rates.
You can see the difference when comparing circular tables or pizza sizes. Doubling a diameter also doubles the radius, so the enclosed area becomes four times larger, not twice as large.
Radius and diameter are not interchangeable
The input is the centre-to-edge distance. If you know the full width across the circle, divide it by two first. Entering a diameter as a radius makes the computed area four times too large.
Area uses squared units: a radius in metres produces square metres, while circumference stays in metres. The tool intentionally uses generic units so you can choose a length system, but every measurement must use the same one.
A few questions, answered
Can I enter the diameter directly?
Divide the diameter by two and enter the resulting radius.
Why is the displayed area approximate?
π is irrational, and the calculator displays a rounded numerical approximation.