Circle Calculator
Enter any one circle measurement to find the radius, diameter, circumference, and area.
Runs in your browser — nothing is sent or saved.
How to use the circle calculator
- Choose which measurement you know.
- Enter its value.
- Read the radius, diameter, circumference, and area.
- Copy the link to save or share it.
One measurement fixes the circle
A circle is defined by a single number, so knowing any one of its measurements gives you the rest. This circle calculator lets you enter the radius, the diameter, the circumference of a circle, or the area, and it works back to the radius and then forward to all four, giving the circle area and circumference in one go. As a diameter calculator it also goes from radius to diameter and back. The area and circumference both grow with the radius, but the area grows faster because it depends on the radius squared.
The circle formulas
The core relationships are short. The diameter is twice the radius. The circumference, the distance around the circle, is 2 times pi times the radius. The area, the space inside, is pi times the radius squared. Pi is about 3.14159, the ratio of any circle's circumference to its diameter. From these three formulas every circle value follows.
A worked example
Take a circle with a radius of 5. The diameter is 2 times 5, which is 10. The circumference is 2 times pi times 5, about 31.42. The area is pi times 5 squared, which is pi times 25, about 78.54. Enter a radius of 5 to check.
FAQ
How do I find the area of a circle?
Square the radius and multiply by pi: area equals pi times r squared. For a radius of 5, that is about 78.54. The tool also derives the radius from any other measurement first.
How do I find the circumference?
Multiply the diameter by pi, or the radius by 2 pi. For a radius of 5, the circumference is about 31.42.
How do I go from diameter to radius?
The radius is half the diameter. Enter the diameter and the tool halves it, then computes everything else.
What is pi?
Pi is the ratio of a circle's circumference to its diameter, about 3.14159. It appears in both the circumference and area formulas.
Is my information private?
Yes. Everything is calculated in your browser and nothing you enter is uploaded or saved.