Circle Calculator
Enter any one value to calculate all circle properties.
This tool is for informational and educational purposes only. It is not a substitute for professional financial, medical, legal, or engineering advice. See Terms of Service.
Can't find what you need?
Request a ToolHow to Use the Circle Calculator
This calculator computes all properties of a circle from a single known measurement. It works in reverse too: if you know the area, it can find the radius.
- Choose the known value type. Select whether you know the radius, diameter, area, or circumference from the dropdown.
- Enter the value. Type the number into the input field. The label updates to match your selection.
- Read all properties. The calculator instantly shows the radius, diameter, area, and circumference. The primary result is the area, displayed prominently at the top.
Results update live as you type. Use the Copy button to grab the result or Share to create a link with your inputs pre-filled.
About Circle Calculations
A circle is defined by its center point and radius. All other properties derive from the radius using pi (approximately 3.14159). The diameter is twice the radius. The circumference (perimeter) is 2 times pi times the radius. The area is pi times the radius squared. These relationships mean that knowing any single property lets you calculate all the others. Circles appear everywhere in engineering, architecture, physics, and daily life, from wheels and pipes to orbits and clock faces.
Frequently Asked Questions
What is the formula for the area of a circle?
The area of a circle is A = pi * r^2, where r is the radius. If you know the diameter d, use A = pi * (d/2)^2. For a circle with radius 5, the area is approximately 78.5398 square units.
How do I find the radius from the area?
To find the radius from the area, use r = sqrt(A / pi). Select "Area" from the dropdown in this calculator, enter the area value, and the radius will be calculated automatically.
What is the relationship between circumference and diameter?
The circumference equals pi times the diameter: C = pi * d. This ratio (C/d = pi) is constant for all circles and is the original definition of pi. It means the circumference is roughly 3.14 times the diameter.