Some gigabytes are bigger than others

I just bought a 500 gigabyte hard drive. When I go to “My Computer” and get the properties of the disk, it reports 465 gigabytes. Where’s the other 35 gigabytes?

A “disk manufacturer’s gigabyte” is not the same as a “computer geek’s gigabyte.” Generally, prefixes like kilo, mega and giga represent successive multiples of 1000. For example, “kilometer” means “1000 meters.” Similarly, mega indicates a million (1000 x 1000 = 1,000,000) and giga, a billion (1000 x 1000 x 1000 = 1,000,000,000). Computers, however, use binary arithmetic (base 2), so it is more convenient to use numbers that are powers of two. Thus, large computer-related quantities are usually expressed as multiples of 1024 (2¹⁰). This means that a kilobyte (KB) is understood to be 1024 bytes rather than 1000 bytes, a megabyte (MB) is 1,048,576 (1024 x 1024) bytes, and so forth.

Hard drive manufacturers had a choice in the early days: They could use either the conventional meanings for these prefixes or the specialized, computing variant. Not surprisingly, they chose the alternative that gave them the bigger numbers. This is why that 500GB hard drive is viewed as a 465GB drive by your computer. A good rule of thumb is to take the advertised drive capacity and subtract a little over 5 percent. For terabyte drives, subtract about 10 percent.

Mind you, the folks who make those drives aren’t really cheating. My 500GB hard drive holds 500,096,991,232 bytes, which is, in fact, a solid 500 gigabytes in the outside world. What can I say? Computer people don’t think like normal people.

For the idly curious and numerically obsessed, here’s a table of the prefixes and their associated multipliers:

By the way, non-geek terminology is used to describe most forms of mass storage, not just hard drives. For instance, the so-called “1.44MB” floppy is really a 1.38MB diskette and the “2GB” flash drive I carry around in my pocket actually stores 1.87GB.