Online Tool Store
Circle Calculator
Enter a circle's radius, diameter, area, or circumference and get the other three calculated instantly. Runs entirely in your browser.
🔒 This tool runs entirely in your browser. Your files are never uploaded to a server.
How it works
- Choose which value you already know — radius, diameter, area, or circumference.
- Type that value in.
- All four values update instantly, calculated from each other.
Every circle measurement can be derived from the radius: diameter = 2r, area = πr², and circumference = 2πr. Whichever value you start from, the calculator first works backward to find the radius, then uses these three formulas to fill in the rest.
FAQ
Which value should I enter?
Whichever one you already know — radius, diameter, area, or circumference. Pick it from the buttons, type the number, and the other three are calculated for you.
What units does the result use?
Whatever unit you entered — enter the radius in centimetres and the area comes out in square centimetres, enter it in inches and the area is in square inches. The calculator just does the arithmetic.
Is anything I enter sent anywhere?
No. Every calculation happens locally in your browser with plain arithmetic — nothing is uploaded or logged.
How is radius found from area or circumference?
From area, radius = √(area ÷ π), which comes from rearranging area = πr². From circumference, radius = circumference ÷ (2π), rearranging circumference = 2πr. Once the radius is known, diameter, area, and circumference all follow directly from it.
Can I use decimals?
Yes — the input accepts decimals, so a radius like 4.5 or an area like 12.75 works exactly as typed.
What happens if I enter 0 or a negative number?
It's rejected — radius, diameter, area, and circumference must all be positive, since none of them can be zero or negative for a real circle.
How we compare
| Feature | Online Tool Store | Basic area calculator | Doing it by hand |
|---|---|---|---|
| No file upload — runs in your browser | ✓ | ✓ | ✓ |
| Start from any of the four values | ✓ | Radius or diameter only | Need to rearrange the formula yourself |
| Shows all four values at once | ✓ | Usually just area | One calculation at a time |
Most area calculators only go one direction — dimensions in, area out — and expect you to already have the radius. Circle Calculator works from whichever measurement you actually have and shows all four values together, so there's no formula to rearrange by hand.