Circle Calculator
Calculate a circle's diameter, circumference, and area from its radius. Free and private, right in your browser.
| Diameter | |
|---|---|
| Circumference | |
| Area |
Type in a circle's radius, diameter, area, or circumference and it instantly works out the other three, handy for homework, cutting round stock to size, sizing a pipe or pizza, or checking a fabrication drawing.
How to use it
- Enter one known value. Radius, diameter, area, or circumference
- Let it calculate the other three instantly
- Copy the results or adjust decimal precision as needed
Frequently asked questions
- How do I find the area if I only know the circumference?
- Enter the circumference and the calculator back-solves for radius (r = C / 2π), then uses that to give you area and diameter automatically, no need to do the algebra yourself.
- What's the difference between radius and diameter here?
- Diameter is just radius × 2. Enter whichever one you have (radius, diameter, area, or circumference) and the tool fills in the other three. You don't need to convert first.
- How many decimal places does it use for pi?
- It uses full floating-point precision for π internally, then lets you choose how many decimal places to display, so rounding error doesn't creep into the other values.
- 1
Nothing is uploaded
The file never leaves your device. There is no server to send it to, so there is nothing to leak, log, or subpoena.
- 2
No signup, no limits
No account, no email, no daily cap, no watermark, and no paid tier waiting behind the third file.
- 3
Works offline
Load the page once and it keeps working with the network off, because all the work was always happening locally.