One hundred twenty gigabytes is a luge amount of music – around 30,000 four minute songs at the ipod classic’s default audio quality, or enough for about 80 days solid listening. It’s also a decent amount of video: about 700 hours at the compression rate the iTunes Store uses. But unfortunately, you don’t actually get the amount of hard disk space that’s written on the iPod or iPhone.

There are two reasons for this, First you lose some hard-disk space to the device’s operating system (OS – the software that enables it to function) and the file allocation table that records which file is stored where on the disk. This happen on all hard disks and solid-state devices that contain operating systems, and costs you anywhere from a few megabytes on the iPod shuffle to a few hundred megabytes on the iPhone.

Second, the capacities of the iPods and the iPhone are measured in “Marketing gigabytes” rather than in real gigabytes. A real gigabyte is 1,024 megabytesm a megabyte is 1,024 kilobytes, and a kilobytes is 1,024 bytes. That makes 1,073,741,824 bytes (1,024*1,024*1,024 bytes) in a real gigabyte. By contrast, a marketing gigabytes has a flat billion bytes (1,000*1,000*1,000 bytes)- a difference of 7.4 percent.

So an iPod or iPhone will actually hold 7.4 percent less data than its listed size suggests(and minus more for the OS and file allocation table). You can see why marketing folks choose to use marketing megabytes and gigabytes rather than real megabytes and gigabytes – the numbers are more impressive. But customers tend to be disappointed when they discover that the real capacity of a device is substantially less than device’s packaging and literature promised.