How to use the Circle Area Calculator
Enter the radius. If you measured the diameter, halve it first.
The circle area formula
Area equals pi times the radius squared: A = πr², with π ≈ 3.14159. For a radius of 5 the area is 3.14159 × 25 ≈ 78.54. Doubling the radius makes the area four times larger, because the radius is squared.
If you know the diameter or circumference
The radius is half the diameter, so a 10-inch pizza has a 5-inch radius. From the circumference C, the radius is C ÷ (2π). Work out the radius first, then enter it above.
The formula
A = πr²
Examples
- Radius 5
Area: 78.53981634
- Radius 12.5
Area: 490.87385212
- Radius 0.75
Area: 1.76714587
Circle area by radius
| Radius | Area |
|---|---|
| 1 | 3.1416 |
| 2 | 12.5664 |
| 3 | 28.2743 |
| 4 | 50.2655 |
| 5 | 78.5398 |
| 6 | 113.0973 |
| 7 | 153.938 |
| 8 | 201.0619 |
| 9 | 254.469 |
| 10 | 314.1593 |
| 12 | 452.3893 |
| 15 | 706.8583 |
| 20 | 1,256.6371 |
| 25 | 1,963.4954 |
| 50 | 7,853.9816 |
| 100 | 31,415.9265 |
Limitations and assumptions
The area comes back in the square of the unit you entered.
Frequently asked questions
What is the area of a circle with radius 10?
About 314.16 square units, because π × 10² = 100π.
How do I find the area from the diameter?
Halve the diameter to get the radius, then use A = πr². Or use A = πd² ÷ 4 directly.
Which unit is the answer in?
The square of the unit you entered. A radius in metres gives square metres; a radius in inches gives square inches.