Circle area calculator
Find the area and circumference of a circle from its radius. Use any consistent length unit.
Result for the values shown.
Calculations run in your browser.
What to enter
| Input | Meaning and units |
|---|---|
| Radius | Enter the radius, which is half the diameter. |
Understanding your result
Read Area (square units), Circumference (units) in the units and model stated on this page. The formula A = πr² · C = 2πr defines the relationship; measurement accuracy and model applicability are separate from numerical precision.
Common mistakes
- Entering the diameter where the radius is requested.
Check your calculation
- Use the worked example as a known reference case, then change one input at a time and check the direction and units of the response.
Calculation checks, sources and review limits
How much area does a circle cover?
Enter the radius to find the circle’s area and circumference. If you know the diameter, divide it by two before entering it.
Common uses
- Measure the area of a circular surface.
- Find the length around a circular boundary.
How it works
Area grows with the square of the radius. Circumference grows linearly. Enter the radius, which is half the diameter. An input in metres gives area in square metres and circumference in metres.
Worked example
A radius of 5 gives an area of approximately 78.5398 square units and a circumference of 31.4159 units.