⭕ Circle Calculator
Enter any one measurement — the other three are calculated instantly.
Everything is derived from the radius
Whichever box you type in, the calculator converts your figure to a radius first, then rebuilds the other three from it:
r = d / 2 = C / (2π) = √(A / π)
d = 2r C = 2πr A = πr²Enter a radius of 5 and you get diameter 10, circumference 31.41593 and area 78.53982. Type that area back in and the radius returns to 5, because √(78.53982 / π) is 5 again. Results are rounded to five decimal places, so a value round-tripped through several fields can drift in the last digit.
Units and what to watch for
- The tool is unitless. Put in inches and the area comes back in square inches; put in metres and it is square metres. Never mix units between fields.
- Area grows with the square of the radius. Doubling the radius doubles the circumference but quadruples the area - a 12 in pizza has more than twice the food of an 8 in one.
Frequently asked questions
How do I find the radius from the circumference?
Divide the circumference by 2π, which is about 6.28319. A circumference of 31.41593 gives a radius of 5.
What is the area of a circle with a 10 inch diameter?
78.53982 square inches. Halve the diameter to get a radius of 5, then square it and multiply by π: 25 × 3.14159 = 78.54.
Why does the calculator fill in the other three boxes?
A circle has one degree of freedom, so any single measurement fixes the whole shape. The tool works out the radius from whatever you typed and writes the remaining values into the other fields.