This free online circle calculator solves for radius, diameter, circumference, and area from any single known value. Just choose what you know - radius, diameter, circumference, or area - enter the value, and instantly get all the other circle measurements.
How to Use the Circle Calculator
Choose "What do you know?" - radius, diameter, circumference, or area.
Enter the "Value" you know in the input field.
Click "Calculate" to see the radius, diameter, circumference, and area.
Review the results table for all four circle measurements at once.
Frequently Asked Questions
Radius is derived first depending on your input: from diameter (r = d/2), from circumference (r = C/2π), or from area (r = √(A/π)). It then computes diameter = 2r, circumference = 2πr, and area = πr².
The calculator requires a positive number greater than zero. If you enter zero, a negative value, or leave the field blank, it shows "Please enter a valid positive number" instead of a result.
If you know the area is 78.54, the calculator finds r = √(78.54/π) ≈ 5, then diameter ≈ 10, circumference ≈ 31.416, and area ≈ 78.54.
Yes, it's completely free with no sign-up required, and it rounds all results to three decimal places for accuracy.