Circle Calculator
Find a circle's area, circumference, and diameter from its radius.
Try an example
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About this calculator
How This Circle Calculator Works
Enter a circle's radius, and the calculator returns its diameter, circumference, and area.
Worked Example
A circle with radius 5 has a diameter of 10, a circumference of about 31.42, and an area of about 78.54.
The Formulas
Diameter = 2r. Circumference = 2πr — the distance around the circle. Area = πr² — the space enclosed by the circle. All three depend on π (pi), the fixed ratio between a circle's circumference and its diameter, approximately 3.14159.
Why Area Grows Faster Than Circumference
Doubling the radius doubles the circumference (a linear relationship) but quadruples the area (a squared relationship), since area scales with r². This is why a pizza twice the diameter isn't twice the food — it's roughly four times as much, which is a useful intuition for comparing sizes of anything circular.
If You Know a Different Measurement
If you know the diameter instead of the radius, divide it by 2. If you know the circumference, divide it by 2π to get the radius first.
Worked example
A circle with radius 5 has a diameter of 10, a circumference of about 31.42, and an area of about 78.54.
Frequently asked questions
What if I only know the diameter or circumference?
Divide the diameter by 2, or divide the circumference by 2π, to get the radius — then enter that into the calculator.
Why does area use r² but circumference uses r?
Area scales with the square of the radius because it measures a two-dimensional space, while circumference is a one-dimensional distance around the edge — this is also why doubling the radius quadruples the area but only doubles the circumference.