⭕ Circle Calculator
Radius5
Diameter10
Area78.5398
Circumference31.4159
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.
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.