⭕ Circle Calculator
Enter just one value — radius, diameter, area, or circumference — and this tool automatically calculates all the others. Handy for DIY, design work, or math homework whenever you need a circle's dimensions quickly.
How to use
- Choose which type of value you're entering (radius, diameter, area, or circumference).
- Enter the value, and every other value for the circle is calculated automatically.
How the calculation works
Enter any one of a circle's radius, diameter, area or circumference and this tool calculates the other three. The input is first converted to the radius r, from which everything else follows: • Diameter = 2r • Circumference = 2πr • Area = πr² From an area, r = √(area ÷ π); from a circumference, r = circumference ÷ 2π. π is JavaScript's Math.PI (3.141592653589793).
Worked example
Radius 5 Diameter: 10 Circumference: 2 × π × 5 ≈ 31.42 Area: π × 5² ≈ 78.54 Radius of a circle with area 100 √(100 ÷ π) ≈ 5.64 A 30 cm pizza has about 707 cm² and a 20 cm pizza about 314 cm²: 1.5 times the diameter gives 2.25 times the area.
Things to be aware of
- Area grows with the square of the radius, so doubling the radius (or diameter) quadruples the area.
- Results use the same length unit you enter; areas are in that unit squared.
- Schools often use π = 3.14 for hand calculations, so the decimals may differ slightly from this tool.
FAQ
Can I work backward from the area to find the radius?
Yes, select "Area" and enter a value — the radius is automatically derived using radius = √(area ÷ π).
How precise is the value of pi used?
It uses JavaScript's built-in Math.PI, which has about 15 digits of precision.
What happens if I enter a negative value?
A circle's size must be positive, so a value of 0 or less won't produce a result.