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Marketing Many people notice missing space right when they open the box. They bought a system with a 160GB hard drive, but Windows shows the total drive capacity as 149GB. That's a difference of 11GB right off the bat. The reason for this has to do with how you measure capacity to begin with. We measure bytes using progressively larger sizes, starting with K (kilobytes), M (megabytes), G (gigabytes), each one standing for a multiple of 1000. So 1K = 1,000, 1M = 1,000,000 (1000 * 1000), and 1G = 1,000,000,000 (1000 * 1000 * 1000). These are the units that companies use when they advertise the size of their disks, so your 160GB drive is 160,000,000,000 bytes in these measurements.
However, computers are binary systems, and measuring in multiples of 1000 isn't the way they do things. The closest thing we have in binary is 1024. So, for a computer, 1K = 1,024, 1M = 1,048,576 (1024 * 1024), and 1G = 1,073,741,824 (1024 * 1024 * 1024). As a result of this, a computer thinks that 1GB is bigger than what a person typically refers to as 1GB (a difference of 73,741,824 bytes). If we take our example of a disk that's advertised as 160GB, and divide by what a computer thinks is 1GB, we wind up with: 160,000,000,000 / 1,073,741,824 = 149.012, which is what Windows says is the drive capacity.
This measurement makes it seem like the drive is smaller, which is the reason I call this "marketing". Everyone wants to make their drives seem bigger, so they use the larger number, even if it's not exactly accurate.
Because of this confusion, new standards of measurement have been devised to help clear this up. Officially, the term "megabyte" refers to 1,000,000 bytes (1000 * 1000), and the term "mebibyte" refers to 1,048,576 bytes (1024 * 1024). The abbreviation for "megabyte" is "MB", like you're used to, and for a "mebibyte" it's "MiB". Notice the "i" in there. It's subtle, but important to make the distinction. You probably won't see these units in use by large companies for a while, but it's something you should be aware of anyway. See
mebibyte for more information.
Here's a table comparing the "marketing" size vs. the computer size for some typical drive sizes:
<caption>Typical drive sizes</caption>
| Marketing |
Computer |
| 80 GB |
74.51 GiB |
| 100 GB |
93.13 GiB |
| 120 GB |
111.76 GiB |
| 140 GB |
130.39 GiB |
| 160 GB |
149.01 GiB |
| 200 GB |
186.26 GiB |
| 250 GB |
232.83 GiB |
| 300 GB |
279.39 GiB |
| 320 GB |
298.02 GiB |
| 350 GB |
325.96 GiB |
| 400 GB |
372.53 GiB |
Problems? See this thread at archive.org.
