It's called "overhead". Part of the problem is space "lost" by the formatting of the drive itself and the data used by the operating system. This is known in the industry as "overhead". The second and most important reason for "missing space" is the way an operating system such as Windows or the Apple OS computes the available space.
"Modern operating systems such as Mac OS X use binary mathematics to define the total capacity of a hard drive. Using binary math, an 80-gigabyte (GB) hard drive reports approximately 74.51 GB of available space.
Operating system compute everything on a drive using binary math. (1's and 0's) In binary math, 1 GB is equal to 1,073,741,824 bytes, whereas conventional (or base 10) mathematics instead calculate 1 GB as exactly 1,000,000,000 bytes."
Using your trusty calculator, you will see that your 500GB drive measured in conventional mathematics has a 500,000,000,000 byte capacity.
The drive size as reported in Binary math is 500000000000 divided by 1,073,741,824 (the number of bytes in a GB in Binary math) which gives 465.661287307739 GB or roughly 465.6 GB
In your case, this 500GB drive would hold 465.6 GB of data which is just enough room for your current requirements. This leaves no room for expansion. Some of the other loss is taken up by the "overhead."