To convert from decimal to binary, you have to sum the negative powers of two corresponding to the positions of the 1s in the binary fraction.
In the case of 0.1101, you need to sum: (1/2 × 1) + (1/4 × 1) + (1/8 × 0) + (1/16 × 1) = 13/16 = 0.8125. Read more