Circle area and circumference calculator
Enter the radius or the diameter to get the area and circumference of a circle. Useful for homework and for measuring round tables, pools and pipes.
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How to use it
- Enter known value, length and length unit.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Area = π × r², circumference = 2 × π × r and diameter = 2 × r, where r is the radius.
Example
Inputs: Known value = Radius, Length = 5, Length unit = Centimeters (cm). Result: Area: 78.5398 cm²; Circumference: 31.4159 cm; Radius: 5 cm.
Limits and caveats
- The full value of π is used, so results differ slightly from answers worked with 3.14 or 22/7.
- The unit choice only labels the results; it does not convert between units.
Questions about this tool
What is the formula for the area of a circle?
Area equals π times the radius squared. A circle with a 5 cm radius has an area of about 78.54 cm².
How do I find the area from the diameter?
Halve the diameter to get the radius and apply the formula, or pick "Diameter" in the calculator and it does that for you.
Why does my answer differ from the textbook?
Many textbooks round π to 3.14 or 22/7. The calculator uses a more precise value, so the decimals can differ a little.