A circle is the set of points at the same distance (the radius) from a centre. Wheels, pipes, plates, round tables and pizza are circles, and the four basic quantities are linked through π ≈ 3.14159.
How to use the Circle Calculator
- Radius r
- Every result field is editable: type into Area or any other result and Radius r plus every other field update to match.
Results update as you type, and everything is calculated in your browser.
How it works
Enter the radius, or type into the diameter, circumference or area field and the radius is worked out backwards; everything else updates at once in the units you choose.
What each result means
- Area = pi×[Radius r]^2
- Diameter = 2×[Radius r]
- Circumference = 2×pi×[Radius r]
Example values
How the main result changes when Radius r changes and everything else stays at the example values above:
| Radius r (cm) | Area (cm²) |
|---|---|
| 2.5 | 19.635 |
| 3.75 | 44.1786 |
| 5 | 78.5398 |
| 6.25 | 122.7185 |
| 7.5 | 176.7146 |
| 10 | 314.1593 |
Formula
d = 2r, C = 2πr = πd, A = πr² = πd²/4 = C²/(4π)
Worked example
Inputs: Radius r: 5 cm.
Results: Area = 78.539816 cm²; Diameter = 10 cm; Circumference = 31.415927 cm.
Key terms
- Radius
- The distance from the centre to the circle.
- Circumference
- The perimeter of a circle.
Frequently asked questions
How do I find the area from the circumference?
A = C²/(4π).
What is π?
The ratio of a circle's circumference to its diameter, ≈ 3.14159.
If I double the radius, what happens to the area?
It becomes four times larger.
What is the difference between radius and diameter?
The diameter is twice the radius, as it goes all the way across.