SHAPES, DATA, AND SCHOOL MATH
Circle Calculator
Calculate the area, circumference, and diameter of a circle using a radius and the standard value of pi.
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Enter your values and select Calculate. Your result stays on this device.
How it works
Area = πr²; circumference = 2πr; diameter = 2r.
Assumptions
The figure is a perfect circle and the radius uses one consistent linear unit.
About this calculator
This circle calculator combines the three most common circle searches in one focused page. Enter the radius once to get area, circumference, and diameter.
How to use it
- Measure or enter the radius from center to edge.
- Keep the radius in the unit you want returned.
- Calculate the area, circumference, and diameter.
- Round only after matching the precision required by your assignment or project.
Use case
circle calculator: a clear planning check
Enter a radius once and calculate πr², 2πr, and 2r together for a transparent geometry check.
Find a circle’s area, circumference, and diameter from one radius.
Method and update
Last reviewed: 2026-08-22. This tool performs the stated formula locally in your browser using the values you enter.
Frequently asked questions
How do I find the area of a circle?
Square the radius and multiply by pi.
How do I find circumference from radius?
Multiply the radius by 2 and by pi.
Is diameter twice the radius?
Yes. The diameter passes through the center and equals two radii.
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