Circle CalculatorFree online math calculator — instant results, no sign-up
Radius, diameter, circumference and area.
About the Circle Calculator
This circle calculator computes the diameter, circumference, and surface area of a circle based on a given radius. It provides quick geometric dimensions for students, fabricators, and designers working with circular shapes.
- Radius
- The distance from the center of the circle to its outer edge, entered in any linear unit of measurement (such as inches, feet, or meters).
Formula and method
Diameter = 2 × r Circumference = 2 × π × r Area = π × r²
- rRadius of the circle
- πPi, the mathematical constant (approximately 3.14159265)
The tool multiplies the radius by 2 to determine the diameter, multiplies the radius by 2π to find the circumference, and squares the radius before multiplying by π to find the area. All output values are rounded to six decimal places.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Radius5
Results
- Diameter10
- Circumference31.415927
- Area78.539816
How to use the Circle Calculator
- 1Enter the known radius of the circle.
- 2Multiply the radius by 2 to obtain the total diameter.
- 3Calculate the circumference by multiplying the radius by 2π.
- 4Calculate the area by squaring the radius and multiplying by π.
Reading your result
The diameter represents the straight-line distance across the circle passing through the center, sharing the same linear unit as the radius. The circumference gives the total perimeter distance around the boundary in linear units, while the area indicates the two-dimensional surface enclosed within the circle in square units.
Limitations
The calculation assumes a perfect, flat two-dimensional Euclidean circle and does not support non-circular ellipses or spherical surfaces. Additionally, the tool does not automatically convert units, so the input and all corresponding outputs share the same base unit system.
Common questions
What units are the circumference and area measured in?+
Circumference is expressed in the same linear unit as the radius (e.g., centimeters or inches), whereas area is expressed in square units (e.g., square centimeters or square inches).
How can I find the radius if I only know the diameter?+
Divide the diameter by 2 to get the radius, then enter that value into the calculator.
What value of pi (π) is used in the calculation?+
The calculation uses standard floating-point precision for π (Math.PI in JavaScript, approximately 3.141592653589793) before rounding the final answers to six decimal places.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.