# Find the area of a circle in C++ :

We can easily find out the area of a circle in C++. The formula to find the circle area is *pi * radius * radius*, where *pi* or *π* is the mathematical constant and *radius* is the radius of the circle.

The value *π* is *3.14159* approximately. In the program, we will take the radius as an input from the user. The user will enter the *radius*, the program will calculate the area and print it out.

With this program, you will learn how to take user inputs and how to do mathematical calculations in C++.

## C++ program to find the area of a circle :

```
#include <iostream>
using namespace std;
int main()
{
// 1
float PI = 3.14159;
float r, a;
// 2
cout << "Enter the radius : " << endl;
cin >> r;
// 3
a = PI * r * r;
// 4
cout << "Area : " << a << endl;
return 0;
}
```

### Explanation :

*The commented numbers in the above program denote the step numbers below :*

- Create one floating-point variable
*PI*to hold the value of*π*. Create two more float variables*r*and*a*to hold the value of*radius*and*area*of the circle. - Ask the user to enter the radius. Read it and store the value in
*r*. - Finally, calculate the area using the formula we have discussed above and store that value in
*a*. - Print out the result to the user.

### Sample Output :

```
Enter the radius :
6
Area : 113.097
Enter the radius :
8
Area : 201.062
Enter the radius :
1
Area : 3.14159
```

## Using cmath header :

If you don’t want to remember the value of *PI*, you can use the *cmath* header library. This library comes with a lot of useful mathematical functions and constants. *PI* is also defined in it. Just import this library using *include* and use *M*PI_ in your program.

We can also find the power of a number using the *pow* method defined in this *cmath* library. In this program, I will use this method to find the value of *radius * radius* :

### C++ program using cmath :

```
#include <iostream>
#include <cmath>
using namespace std;
int main()
{
float r, a;
cout << "Enter the radius : " << endl; cin >> r;
a = M_PI * pow(r,2);
cout << "Area : " << a << endl;
return 0;
}
```

Looks cleaner. Try to use headers instead of doing the logical calculations on your own.

### Output :

The output will be the same as the previous example.