# PI Constant in C++

In this tutorial, we will learn how to use the PI constant in C++.

PI is a mathematical constant which we use in various mathematical calculations ranging from simple finding area of a circle to more complex Stoke’s theorem in mathematics. C++ has a predefined constant in its math library which we can use to access the value of pi wherever needed in our program.

We use the following header file :

```#define _USE_MATH_DEFINES
#include <cmath>```

Here, _USE_MATH_DEFINES is a #define macro. Later in the program, we use M_PI to access the value of PI. In C++, the value of M_PI is 3.14159265358979323846.

Below I write a simple program to print the value of PI.

## Print the value of PI in C++

```#define _USE_MATH_DEFINES
#include<cmath>
#include<iostream>
using namespace std;
int main()
{
cout << "value of pi is = " << M_PI << endl;
return 0;
}```

Output:

`value of pi is = 3.14159`

Now that we know how to use the constant, let’s find out the volume of a sphere with radius r. In this program, we use the formula for the volume of a sphere which is 4/3 π r^3.

### Program to find the volume of a sphere in C++

```#define _USE_MATH_DEFINES
#include<cmath>
#include<iostream>
using namespace std;
int main()
{
int r;
float v;
cout << "Enter the value of radius : ";
cin >> r;
v = float(4) / float(3) * M_PI * r * r * r;
cout << "volume of the sphere is : " << v << endl;
return 0;
}```

Output :

```Enter the value of radius : 4
volume of the sphere is : 268.083```

You can also always define your own value of pi and use it in a program.

## Set Value of PI constant in C++

```#include<iostream>
using namespace std;
const double PI = 3.141592653589793238463;    //value of pi
int main()
{
cout << "value of pi is : " << PI;
}```

Output :

`value of pi is = 3.14159`

Hope this was helpful. Enjoy Coding!