Circle Calculator FAQs
What can this circle calculator find?
This circle calculator can find radius, diameter, circumference, and area from any one known value. Enter one value, choose the unit, and the remaining circle values are calculated immediately.
What is the formula for the area of a circle?
The formula for the area of a circle is A = pi r^2. If you know the diameter instead, you can use A = pi d^2 / 4.
How do I find diameter from circumference?
Use d = C / pi. The calculator also shows this inverse formula automatically when circumference is the selected input.
How do I find radius from area?
Use r = sqrt(A / pi). This is useful when you know the area of a circle but need the radius for geometry, construction, or design work.
Example: what are the diameter, circumference, and area of a circle with radius 10 cm?
If the radius is 10 cm, the diameter is 20 cm, the circumference is about 62.832 cm, and the area is about 314.159 cm^2. This is a simple example of using one known circle value to calculate the others.
