Bitwise NOT is a bit-level operation on a single binary value that sets the bits that are off and turns off the bits that are on. For example, performing a bitwise NOT on 163 gives 92.

Performing a bitwise NOT on a number $n$ has the same effect as performing a bitwise XOR on a Mersenne number of the form $2^{k}-1$ (where $k$ is the bit size of the data type in use, e.g., 8 for bytes, 16 for words, 32 for double words, etc.) and $n$. Obviously a bitwise NOT on 0 gives the largest Mersenne number that can fit in the data type in use.

The Windows Calculator offers bitwise NOT in scientific calculator mode, while the Mac OS X Calculator offers it in programmer mode.