Enter dimensions
Results
Enter your dimensions above and the results appear here.
About the Circle Calculator
The circle calculator is a free online tool: area, circumference, radius and diameter. Enter radius (r) and the results update instantly, alongside a labelled diagram so you always know which measurement goes where.
Choose your input unit (mm, cm, m, km, in, ft or yd) and optionally show the results in a different one: lengths, areas and volumes are converted correctly. Every answer shows the formula behind it, and the calculation runs in your browser, so nothing is uploaded.
Circle Calculator example
Radius (r) = 5 cmArea = 78.5398 cm²
Circumference = 31.4159 cm
Diameter = 10 cmHow to use the Circle Calculator
- Pick the unit your measurements use. All dimensions must share the same unit.
- Enter radius (r). Focus a field to highlight the matching label on the diagram.
- Read the results as you type. The largest card is the main answer; the others are the related measurements.
- Copy a single value, copy everything, or share a link that keeps your inputs.
Circle Calculator FAQ
What formulas does the circle calculator use?
A = πr²; C = 2πr; d = 2r.
How do I find the area of a circle from its diameter?
Halve the diameter to get the radius, then use A = πr². Or enter the diameter ÷ 2 as the radius above and read the area directly.
Can I use different units?
Yes. Pick the input unit and, if you like, a different output unit. Area results are shown in squared units and volumes in cubed units.
Is this circle calculator free and private?
Yes. It is free with no sign-up, and calculations happen on your device, so your numbers are never sent to a server.
More 2d shapes
Sector & Arc CalculatorRing (Annulus) CalculatorEllipse CalculatorSquare CalculatorRectangle Calculator