Math
Cylinder
Calculate cylinder volume and surface area, or solve a missing radius, diameter, or height from volume.
Calculators / Math / Circle
Circle calculator
Calculate area, circumference, and diameter.
On this page
Area78.54square units
Circumference31.42
Diameter10
Calculator controls
How to calculate
Calculate area, circumference, and diameter.
Area = πr²; circumference = 2πr
Worked example
Radius: 5.
Area: 78.54 square units; Circumference: 31.42 ; Diameter: 10 .
Assumptions and limits
Use any length unit. The area is in its square units.
Results are rounded for display; calculations use unrounded values. Read our calculation methodology.
Understanding the result
Area measures the enclosed surface, circumference measures the boundary, and diameter is twice the radius. These quantities have different units.
When this tool is useful
Estimate the size or perimeter of a circular opening or surface.
Understanding your inputs
| Input | What to enter |
|---|---|
| Radius | Enter a number of at least 0.000001 and no more than 1000000000000. |
Frequently asked questions
Should I enter radius or diameter?
Enter radius, which runs from the centre to the edge. If you measured the full diameter, divide it by two first.
Why does doubling the radius quadruple the area?
Area contains r². Replacing r with 2r gives 4r², while circumference only doubles because it is proportional to r.