How to Find a Circle's Diameter, Circumference and Area From Its Radius
Three formulas describe a circle completely, and all three start from the radius. This guide explains each one.
What is the Circle Calculator?
Enter the radius, the distance from the centre to the edge. The diameter is twice the radius, the circumference is two times pi times the radius, and the area is pi times the radius squared. One measurement therefore gives you the whole circle.
How to use it
- Enter the radius.
- Keep the unit consistent with whatever you are measuring.
- Calculate to see the diameter, circumference and area.
Worked example
Radius 5 gives diameter 10, circumference 31.42 and area 78.54.
What is the circumference and area of a circle with radius 5?
The diameter is 10, the circumference is 2 times pi times 5, which is about 31.42, and the area is pi times 25, which is about 78.54. So the circle is about 31.42 units around and covers about 78.54 square units.
Frequently asked questions
What is the difference between radius and diameter?
The radius goes from the centre to the edge, while the diameter goes all the way across through the centre, so the diameter is always twice the radius.
What value of pi is used?
The calculation uses the full precision value of pi built into the browser, and the displayed answers are rounded to two decimal places.