Circle Calculator
Any circle measurement from one value.
- 100% free
- No sign-up
- Private — runs in your browser
- Instant results
Everything about a circle from one number
A circle is fully defined by a single measurement. Give this calculator any one of the radius, diameter, circumference, or area and it works out the other three instantly using π (pi ≈ 3.14159).
The formulas
- Diameter = 2 × radius
- Circumference = 2 × π × radius = π × diameter
- Area = π × radius²
FAQ
How do I go from area back to radius?
Rearrange the area formula: radius = √(area ÷ π). This tool does it automatically when you choose "Area" as the known value — handy when you know how much space a circle covers but not its size.
Is this circle calculator free to use?
Yes, it is completely free with no sign-up or limits. Enter any one value and get the radius, diameter, circumference, and area as many times as you need.
What value of pi does it use?
It uses the full-precision pi value built into JavaScript (about 3.14159265358979), not a rounded 3.14. That keeps results accurate to many decimal places for engineering or geometry work.
What units does it use?
The calculator is unit-agnostic: whatever unit you put in comes out the same way. Lengths share that unit, and area is in the squared version of it — so a radius in cm gives area in cm².
Does my data leave my device?
No, every calculation runs locally in your browser. Nothing you type is uploaded or saved, and the tool works on phones, tablets, and desktops alike.