About circle calculations
This calculator works out everything about a circle from any single value. Enter the radius, diameter, circumference or area and it instantly gives you all the others using r = d/2, C = 2πr and A = πr². It runs in your browser.
How Circle works
How to use it
- Enter any one of radius, diameter, circumference or area.
- Read all the other values.
The formulas
Radius is half the diameter; circumference C = 2πr; area A = πr². From any one value the tool finds the radius first, then derives the rest.
Common uses
- Find the area of a circle
- Calculate circumference
- Convert radius and diameter
- Work out a circle from its area
- Help with geometry homework
- Plan circular layouts
- Calculate pipe or wheel sizes
- Learn circle formulas
Frequently asked questions
How do I find the area of a circle?
Enter the radius (or any value) and read the area, A = πr².
What is the circumference formula?
C = 2πr (or πd).
Can I enter the area to get the radius?
Yes — enter any single value and the rest are derived.
Is my input uploaded?
No — it calculates in your browser.
What value of π is used?
The full precision π from your browser.
Does it round?
Results are shown to several decimal places.
Is it free?
Yes — completely free with no sign-up.