Geometry · Circle
Circle Calculator
Enter any one measurement of a circle to find the rest.
Circle Calculator
Choose which measurement you know, enter its value, then select Calculate.
How these calculations work
The diameter is twice the radius. The circumference is 2π times the radius (or π times the diameter). The area is π times the radius squared. Since each of these four measurements determines the other three, entering any one is enough to find the rest.
Examples
A circle with a radius of 5 has a diameter of 10, a circumference of about 31.42, and an area of about 78.54. A circle with an area of 100 has a radius of about 5.64.
About these formulas
These are standard geometry formulas — no external source citation is needed for them.
Related tools
More calculators are on the way — browse everything planned on the Home page.
Frequently asked questions
What units does the result use?
Whatever unit you enter your measurement in — the circumference and diameter are in that same unit, and the area is in that unit squared.
Why does the calculator say my input is invalid?
The field is empty or not a number, or the value is zero or negative — a radius, diameter, circumference, or area must be a positive number.
Circle Solution
Radius
Result will appear here.
Calculation steps