SI and binary prefixes
Some common capacities often advertised and what their "real" size is.
|Advertised size (what
the package says)
|Real size (what your
system* tells you)
|4.7 GB (single-layer DVD)||4.37 GiB|
|8.5 GB (dual-layer DVD)||7.9 GiB|
|50 GB (Blu-Ray)||46.6 GiB|
|320 GB (hard drive)||298 GiB|
|500 GB (hard drive)||465.7 GiB|
|750 GB (hard drive)||698.5 GiB|
|1 TB (hard drive)||931 GiB (0.9 TiB)|
|1.5 TB (hard drive)||1397 GiB (1.36 GiB)|
|2 TB (hard drive)||1862 GiB (1.82 TiB)|
CD-R(W) discs are sold in two sizes, 650 MB and 700 MB. For some reason, the 650 MB really is 650 MiB, but the 700 MB disc is actually 703 MiB.
As usual, Wikipedia has a good article about the whole vile mess. Read it and weep.
*) If your system does not tell you this (or gets the numbers wrong), then your system is broken (no excuses).