Circle Calculator
Calculate the radius, diameter, circumference, and area of a circle by entering any single known positive measurement.
Run tool
This online circle calculator computes the radius, diameter, circumference, and area of a circle from any single known measurement. When you enter a positive number into one of the four available input fields, the calculator derives the other three remaining geometric properties using standard circle formulas based on pi. Each calculated numeric value in the output is rounded to six decimal places for precise reading.
The tool requires exactly one positive measure to perform the calculation. You can provide a radius, a diameter, a circumference, or a total surface area. If all input fields remain zero, or if two or more positive values are entered simultaneously, the tool returns an error notification prompting you to enter only one value. Negative numbers and zero values are ignored and do not count toward selecting the active calculation dimension. The calculation updates immediately once you submit your chosen dimension.
Instructions
- Choose which measurement you know: radius, diameter, circumference, or area.
- Enter a positive number into that specific field and leave all other fields as zero.
- Submit the form to compute all circle dimensions.
- Review the resulting radius, diameter, circumference, and area rounded to six decimal places.
Examples
- Entering a radius of 5 calculates a diameter of 10, a circumference of 31.415927, and an area of 78.539816.
- Entering an area of 50 calculates a radius of 3.989423, a diameter of 7.978846, and a circumference of 25.066283.
FAQ
Can I enter more than one measurement at a time?
No. The calculator requires exactly one positive measurement. If multiple positive values are provided, the tool prompts you to keep only one field filled.
How are the calculated values rounded?
All calculated circle values are rounded to six decimal places.
What happens if I enter zero or negative numbers?
Zero and negative numbers do not count as valid inputs. If no field contains a positive value, the tool prompts you to enter a positive measure.