Circle Calculator
Choose what you know about the circle, enter its value, and get the radius, diameter, circumference, and area.
How circle formulas work
All of a circle's measurements can be derived from its radius (r): diameter = 2r, circumference = 2πr, and area = πr². This calculator works backward from whichever value you know to find the radius first, then calculates the rest from there.
π (pi) is approximately 3.14159 — the ratio of a circle's circumference to its diameter, which is the same for every circle regardless of size.
Frequently asked questions
How do I find the area of a circle from its diameter?
First find the radius by dividing the diameter by 2, then use area = πr². For example, a diameter of 10 gives a radius of 5, so the area is π × 5² ≈ 78.54.
How do I find the radius from the circumference?
Divide the circumference by 2π. For example, a circumference of 31.4 gives a radius of 31.4 / (2π) ≈ 5.
What's the difference between circumference and perimeter?
Circumference is simply the term used for the perimeter of a circle — the total distance around its edge.