β Circle Calculator
Enter any one known value β radius, diameter, circumference, or area β and instantly calculate all the others.
Known Value
Your Result
All results use the same unit as your input (e.g. if you entered radius in cm, area will be in cmΒ²).
How the Circle Calculator Works
A circle is fully defined by a single measurement. Once you know the radius, diameter, circumference, or area, every other property can be derived using Ο (pi β 3.14159).
Formulas
Circumference = 2 Γ Ο Γ radius
Area = Ο Γ radiusΒ²
Worked Example
The calculator loads with a radius of 5. The diameter is 2 Γ 5 = 10, the circumference is 2 Γ Ο Γ 5 β 31.416, and the area is Ο Γ 5Β² β 78.54. All four values update instantly whenever you change the known value or switch which measurement you're entering.
Frequently Asked Questions
How do I find the radius from the area?
Rearrange the area formula: radius = β(area / Ο). This calculator does that automatically when you enter area as the known value.
What's the difference between circumference and area?
Circumference is the distance around the circle (its perimeter), measured in linear units like cm or in. Area is the space inside the circle, measured in square units like cmΒ² or inΒ².
What value of Ο does this calculator use?
It uses JavaScript's built-in Math.PI, which is accurate to about 15 decimal places β far more precise than the common approximation 3.14.