Circle Calculator
Enter the radius to find a circle's diameter, circumference and area.
Reviewed by the WorldCalcs team · Methodology · Last reviewed: June 2026
Diameter
10
d = 2r
Circumference
31.4159
C = 2πr
Area
78.5398
A = πr²
What is a circle?
A circle is the set of all points the same distance from a centre point. That distance is the radius, and twice the radius — straight across through the centre — is the diameter. The distance around the circle is its circumference, and the space inside it is the area. These four measures are all linked by the constant π (pi), about 3.14159.
The circle formulas
From the radius r: the diameter is d = 2r, the circumference is C = 2πr (or πd), and the area is A = πr². Pi (π) is the ratio of any circle's circumference to its diameter, and it is the same for every circle.
Example
For a circle with radius 5, the diameter is 2 × 5 = 10, the circumference is 2 × π × 5 ≈ 31.4159, and the area is π × 5² ≈ 78.5398.
All calculations happen in your browser. Nothing is sent, stored, or tracked.
Results are estimates and may contain errors — for general information only, not professional advice. Always verify before relying on them. Disclaimer
How to use
Type a positive radius. The calculator returns the diameter (2r), circumference (2πr) and area (πr²) instantly.
Values are rounded to about 6 significant digits using the standard constant π ≈ 3.14159.
Frequently asked questions
What is the formula for the area of a circle?+
How do you find the circumference?+
What is the difference between radius and diameter?+
What is pi?+
How do you find the radius from the area?+
Related calculators
- Pythagorean TheoremFind the hypotenuse or a missing leg of a right triangle.
- Triangle Area CalculatorArea from base and height, or from three sides with Heron's formula.
- Rectangle CalculatorArea, perimeter and diagonal from length and width.
- Cylinder CalculatorVolume and total surface area from radius and height.