Circle Calculator
Calculate a circle's radius, diameter, circumference, and area from one known measurement.
Data handling
Storage and export controls
- Retention
- The tool's working state is kept only while this page is open. Inputs and results are not saved to persistent storage by Got A Tool.
- Export
- The tool may prepare a result after you run it. It downloads or copies it only when you choose that action.
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How it works
Circle Calculator: a practical guide
A circle calculator derives a circle's radius, diameter, circumference, and area from one known measurement. It uses the same unit you enter, so a radius in centimeters produces an area in square centimeters and a circumference in centimeters.
- STEP 01
Choose whether the number you know is a radius, diameter, or circumference.
- STEP 02
Enter that positive measurement in any consistent unit.
- STEP 03
Calculate to review the other measurements and area before rounding for your own use.
What this tool does and its limits
The tool uses diameter = 2 × radius, circumference = 2π × radius, and area = π × radius². Results are numerical estimates displayed to a practical number of decimal places. It does not measure from an image, convert units, estimate partial circles, or account for real-world tolerances in manufactured objects.
Inputs, selected files, and results stay in your browser. The tool does not upload them to Got A Tool or save them to persistent storage.
Questions
Circle Calculator details
What is the difference between radius and diameter?
The radius runs from the center to the edge. The diameter runs across the circle through the center and is twice the radius.
What unit should I use?
Use any unit, but keep it consistent. If you enter centimeters, circumference is in centimeters and area is in square centimeters.
Why does area use square units?
Area measures two dimensions. A circle with a radius measured in inches has an area measured in square inches.
Can this calculate a semicircle?
No. This page calculates a complete ideal circle. Apply the appropriate formula separately for arcs, sectors, or partial circles.
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