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  Windows Reports Lower Disk Capacity Than Manufacturer (Disk Inflation By the Numbers)

I just bought a 3.1 gigabyte hard drive. When I go to My Computer and get the properties of the disk, it reports 2.93 gigabytes. Where's the other 170 megabytes?

A "disk manufacturer's gigabyte" is not the same as a "computer 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. Which is why that 3.1GB hard drive is viewed as a 2.93GB drive by your computer. A good rule of thumb is to take the advertised drive capacity and subtract 5 percent. If terabyte drives ever hit the market, subtract 10 percent.

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

Prefix   Conventional   Computing
kilo   1000   1024
mega   1,000,000   1,048,576
giga   1,000,000,000   1,073,741,824
tera   1,000,000,000,000   1,099,511,627,776

By the way, the "thin megabyte" is used to describe most forms of mass storage. For instance, the so-called "1.44MB" floppy is really a 1.38MB diskette and a "100MB" Zip disk actually stores 95.7MB.


Maintained by William K. Walker
Copyright 1997 by William K. Walker
Last update: 22 Jan 97