Circle Calculator — Radius, Diameter, Circumference and Area
One measurement in, all four out.
Runs on your device What you paste is processed by your own browser. It is never sent to a server, and it is not stored anywhere after you leave the page.
Short answer
A circle can be worked out for free in a web browser from any one of its four measurements: give the radius, the diameter, the circumference or the area, and the other three follow from C = 2πr and A = πr². Several circles can be calculated at once, one per line, and every result is shown to twelve significant digits unless a fixed number of decimal places is chosen.
At a glance
| Accepts | One measurement per line |
|---|---|
| Produces | Radius, diameter, circumference and area |
| Processing | In your browser |
| Price | Free, no limits |
| Account | Not required |
Which measurement you have is whichever one was easiest to take, and it is rarely the radius: a pipe is sold by its diameter, a tree is measured around its trunk, and a plot of land arrives as an area. So the dropdown says what the number is, and the page works back to the radius from there.
π is the double-precision value, correct to about sixteen digits — far finer than anything that went into a measurement taken with a tape. Results print to twelve significant digits, which is already more than the input can justify; pick a fixed number of decimals for something a joiner can work to.
How to use it
- 1
Say what your number is
Radius, diameter, circumference or area.
- 2
Type the measurements
One per line. Several circles at a time is the point of the box.
- 3
Convert
Each line comes back as a table of all four measurements.
- 4
Copy the result
Copy to the clipboard, or download it as a text file.
Frequently asked questions
Which measurement should I choose?
The one you actually have: the diameter for a pipe or a wheel, the circumference for anything measured with a tape around it, the area for a plot. The other three are derived either way, so there is nothing lost.
What units does it work in?
Whichever are yours, consistently: a radius in centimetres gives a circumference in centimetres and an area in square centimetres. No unit conversion happens here — the unit converter does that.
How do I get the area from the circumference?
A = C² ÷ 4π. This page gets there by way of the radius, C ÷ 2π, which is the same relationship arranged differently.
Does it handle a semicircle, a sector or a ring?
No, these are whole-circle figures. Half the area is a semicircle; a sector is the area times its angle over 360; a ring is the larger area minus the smaller, which is two lines of this tool and a subtraction.