Compute dynamic range in a linear quantization system
Just to summarize the comments, for the next person who comes looking:

decibels are $20 \log_{10}{x}$ (or $10 \log_{10}{\left( x^2 \right) }$ when power levels used instead of the amplitude levels)

When looking at bit noise limited N-bit system, there are $2^{N}$ measurement levels, and the smallest distinguishable difference is $1$. So

DR$_{dB}$ = $20 \log_{10}{\frac{2^N}{1} } = N*20\log_{10}{2}\approx 6 N$