Bitmap size in memory

In the most of the cases, if you're running more than 256 colors, it will be 32 bits (one processor word, even if the color itself is represented by 3 bytes of the four).
Otherwise, for 256 colors, 1 byte;
and for 16 colors 1 bit of four different bytes (one per color plane, also named RGBI)

The other thing to remember is that 24 bit images are word aligned, so there can be some padding at the end of each row. This means that the size is not per pixel at all. Perhaps if you told us what you're doing, we could be more precise in our replies.