To Convert Binary Numbers into Decimal , we need to multiply each binary digit with Powers of 2 starting from right side.

For Example. Binary Number 100101 will be converted to Decimal using following steps

(2^5) * 1 + (2^4) * 0 + (2^3) * 0 + (2^2) * 1 + (2^1) * 0 + (2^0) * 1

Power of 2 is starting from 0 and increasing from righ to left.

= 32+0+0+4+0+1 = 37

So 100101 is Equal to 37 in Decimal System

**Code for Binary to Decimal:**

```
//Binary to decimal
#include<iostream>
#include<cmath>
using namespace std;
int main()
{
char arr[100];
long int dec=0,i,j;
cout<<"Enter binary "<<endl;
cin>>arr;
for(i=0;arr[i]!='\0';i++);
for(i=i-1,j=0;i>=0;i--,j++)
{
dec+=(arr[j]-48)*pow(2,i);
}
cout<<"\nDecimal "<<dec;
}
```

**Output:**

```
Enter binary
100101
Decimal 37
```