Quick start
- Choose radius, diameter, circumference, or area mode.
- Enter the known value.
- Add a unit label if useful.
- Press Calculate circle to find radius, diameter, circumference, and area.
Best uses
Start here if one of these sounds like your job. The examples below show which inputs matter most.
- Convert radius into diameter, circumference, and area.
- Work backward from diameter, circumference, or area.
- Check circle formulas for geometry class or quick planning.
- Copy circle measurements and formula steps into notes.
Circle formulas used
The calculator uses d = 2r, C = 2pi r, and A = pi r^2.
When you start from diameter, circumference, or area, it rearranges the formula to find radius first.
Choosing a starting value
Use radius mode when you know the distance from the center to the edge.
Use diameter mode when you know the full width through the center. Use circumference or area mode when those measurements are already known.
Reading units
Radius, diameter, and circumference use length units. Area uses square units.
If you enter cm as the unit label, the area result is shown as cm^2.
Worked examples for Circle Calculator
Area = 78.5398163397
Radius = 5
Radius is about 5
FAQ in plain language
What circle measurements can I start with?
You can start with radius, diameter, circumference, or area. The calculator finds the remaining circle measurements.
What formulas does the Circle Calculator use?
It uses d = 2r, C = 2pi r, and A = pi r^2. It rearranges those formulas when you start from diameter, circumference, or area.
What do the main Circle Calculator inputs mean?
The main inputs are the numbers, operation, mode, or known values the calculator needs. Keep units consistent, enter percentages the way the page label shows, and use the examples as a quick check before trusting the answer.
How should I read the Circle Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the Circle Calculator?
Check units, signs, rounding, and the selected mode before copying the answer. If the number feels weird, rerun one of the examples first, then put your own values back in slowly.
What is the difference between radius and diameter?
Radius is the distance from the center to the circle edge. Diameter is the full distance across the circle through the center, so diameter is twice the radius.
Can I enter area to find radius?
Yes. Area mode uses r = sqrt(A / pi) to work backward from a known area.
Sources
Use these if you want to compare the formula, inputs, or limits with a trusted outside explanation.
Related tools
- Area Calculator Calculate flat-shape area for rectangles, triangles, circles, trapezoids, and parallelograms.
- Surface Area Calculator Calculate surface area for boxes, cubes, cylinders, spheres, and cones.
- Volume Calculator Find cubic volume for boxes, cubes, cylinders, spheres, and cones.
Keep exploring
If this guide is close but not exact, these links keep you near the same kind of problem.
- Calculators Browse the full category for related tools that help with the same job.
- All free tools Search the complete Access Free Tools library by task, category, or tool name.
- All calculator and utility guides Find more plain-language examples, formulas, mistakes, and result explanations.
- Free calculator resources Start here when you are not sure which calculator page fits.
Privacy and copying results
Recent answers stay visible only while you work in the current browser tab. They are not sent to a server.
Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.