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Circle Calculator
Know one measurement of a circle? Pick radius, diameter, circumference or area, type the value and get the other three right away, with the formulas worked out and the results converted to other units.
I know the
For area, enter square units (e.g. cm²)
Enter your values to see the result.
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How to use the Circle Calculator
- Choose what you know: radius, diameter, circumference or area.
- Type the value. For area use square units.
- Pick the unit of length.
- Read the results: the other three measurements appear instantly.
- Copy the results or download the table in other units.
Circle formulas at a glance
| You know | Radius | Diameter | Circumference | Area |
|---|---|---|---|---|
| Radius r | r | 2r | 2πr | πr² |
| Diameter d | d ÷ 2 | d | πd | πd² ÷ 4 |
| Circumference C | C ÷ 2π | C ÷ π | C | C² ÷ 4π |
| Area A | √(A ÷ π) | 2√(A ÷ π) | 2√(πA) | A |
Reference values
| Radius | Diameter | Circumference | Area |
|---|---|---|---|
| 1 | 2 | 6.2832 | 3.1416 |
| 2 | 4 | 12.5664 | 12.5664 |
| 3 | 6 | 18.8496 | 28.2743 |
| 5 | 10 | 31.4159 | 78.5398 |
| 6 | 12 | 37.6991 | 113.0973 |
| 8 | 16 | 50.2655 | 201.0619 |
| 10 | 20 | 62.8319 | 314.1593 |
π (pi) is the ratio of any circle's circumference to its diameter, about 3.14159. It is irrational, so circle results are usually rounded; use more decimals when small errors matter, for example in machining.
Frequently asked questions
What are the circle formulas?
With radius r: diameter d = 2r, circumference C = 2πr (or πd), and area A = πr². Going backwards: r = d ÷ 2, r = C ÷ 2π and r = √(A ÷ π).
How do I find the radius from the circumference?
Divide the circumference by 2π (about 6.28319). A circumference of 31.4159 cm gives a radius of 5 cm and a diameter of 10 cm.
How do I find the diameter from the area?
First find the radius with r = √(A ÷ π), then double it. An area of 100 cm² gives r = √(31.831) ≈ 5.642 cm and d ≈ 11.284 cm.
What value of pi does the calculator use?
It uses JavaScript's Math.PI, which is π to about 15–16 significant digits (3.141592653589793). Results are rounded to 4 decimal places for display.
Is a 16-inch pizza twice as big as a 12-inch pizza?
Not quite: area grows with the square of the diameter. A 12-inch pizza has about 113.1 in² and a 16-inch pizza about 201.1 in², so the 16-inch one is roughly 1.78 times larger.
What is the difference between circumference and perimeter?
They mean the same thing: the distance around the outside. 'Circumference' is the word used for circles and ellipses, 'perimeter' for polygons.
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