Contrary to popular belief, this anomaly is not caused by factory-installed programs, file system overhead, or swap space… it’s about math and marketing.
Hard drives are sold and marketed using decimal gigabytes. That is, a “GB” consists of 1,000,000,000 bytes.
However, computers interpret gigabytes in binary. To a computer, 1 GB = 2^30 bytes, or 1,073,741,824 bytes.
The ratio of “actual” to “marketed” file size is the ratio of these two interpretations, or roughly 0.9313225.
Therefore an X-sized (marketed) drive actually has 0.9313225*X of space usable to a computer.
Ex: (iPod Sizes)———————————————————————————
60GB*0.9313225 = 55.88GB
40GB*0.9313225 =37.25 GB
30GB*0.9313225 = 27.94GB
20GB*0.9313225 = 18.6 GB
15GB*0.9313225 = 13.97GB
10GB*0.9313225 = 9.31GB
6GB*0.9313225 = 5.59GB
5GB*0.9313225 = 4.67GB
4GB*0.9313225 = 3.73GB
1GB*0.9313225 = 0.93GB
512MB*0.9313225 = 476.84MB
(Only the much smaller remaining difference is due to these things… like the 37.25 vs. the 37.1GB value that a 40GB iPod reports.)