spmug 596 Adding Up Drive Capacity Q. I have two 250-gigabyte external
drives that I use with my PowerMac. When I open the Mac's System
Profiler program, it says that the drives are 232 gigabytes each. I'm
wondering why this discrepancy exists. A. The discrepancy you see
between the advertised capacity of the hard drive and what the computer
reports reflects different mathematical measurement systems being used
to calculate the size. Computer operating systems use binary math to
measure capacity, in which a kilobyte is not 1,000 bytes but 1,024 bytes
- or 210. Hardware manufacturers, on the other hand, use the base 10
system. So while a hard drive may be advertised by its maker as having a
capacity of say, 80 gigabytes (on the base 10 system), the computer will
report the same drive as having a capacity of about 74 gigabytes in the
binary math system. Breaking it down into bytes shows the discrepancy
more clearly: the drive manufacturer is counting one gigabyte as one
billion bytes (1,0003) and the computer is counting that same one
gigabyte in binary as 1,073,741,824 bytes (1,0243). As drives get
bigger, the discrepancy gets larger. (An explanation of binary math is
at kb.iu.edu/data/ackw.html.) The hard drives on new computers may also
show less available space than advertised. In addition to the differing
math systems calculating the size of the drive, many new computers ship
with a significant amount of installed software, including the operating
system, free and sample programs and other applications that take up
room.