Converting binary to decimal requires knowing the position value of each bit.

The right most bit is position 0:

For an 8 bit binary value

        Binary:      0  1  0  0  1  0  1  1
      Position:      7  6  5  4  3  2  1  0
Position Value:    128 64 32 16  8  4  2  1
   

The value of bit position k is 2^k

The decimal value of binary 01001011 is the sum of the positions values of the non-zero bits: 64 + 8 + 2 + 1 = 75