Yes, that is about right. When a disk if formatted some space is lost.
Fred Langa explains it well:
Hi Fred: Why is there a discrepancy between the size of a 120 Gb. hard disk and the size reported by Windows Pro that claims 111.8 Gb.? What has happened to the other 8.2 Gb.?
The explanation is that hard drive manufacturers calculate hard disk size in 'base 10' notation while Windows does the calculation in 'base 2' (binary) format. This is applicable, both the manufacturer and Windows are right, OK?
But I have read information about something different, for example: hard disk manufacturers use a 160 Gb. drive that has errors on it and sell it like a brand new 120 Gb., because in their line of products they don't have a 140 Gb. You can compare it with chip makers and their processors: you buy a 1800 Mhz. but with overclocking you maybe can reach 2000 Mhz. or more. They guarantee that the processor works well at 1800 Mhz.
Using the same example, can one access the other "good" parts of the remaining 40 Gb. that are hidden with special software or tools that people can't reach? What is the truth? Thanks in advance for your reply. ---Enrique Paff
All the factors you mention can come into play, Enrique, and we'll discuss them in a moment. But there's one more that has an even larger effect: Drives are sold by "raw" or unformatted capacity. When you partition and format a drive, some of the space on the drive is occupied by the partitioning and formatting data structures.
By analogy: think of a filing cabinet. As sold, it can hold a certain number of pages per drawer. But when you add hanging folders, the frames for the folders, index pages, and so on, you actually lose a little space, but can then organize and find your papers more easily. It's the same with hard drives: The partitioning and formatting takes up some space on the drive, but is necessary to organize and find your files.
Older drive formats (FAT, FAT16, the early Linux formats, etc), were created in the days when drives were tiny compared to today's. The older formats are not very efficient, and can waste a huge amount of space on a large drive. Those formats also can have severe, built-in limits to the size of the drives or partitions they can "see;" today's drives can simply be beyond the ability of these older formats to handle well.
Newer formats (FAT32, ext2, etc.) do better with larger drives; and some formats (NTFS, ext3, ReiserFS, etc.) were specifically designed with very large drives in mind. These latter formats help you to make the most of your disk space, with minimal wastage and no practical limits on disk or partition size. (Yes, there are limits--- e.g. 2 terabytes for NTFS--- but most of us won't reach them anytime soon. <g>)
More info on formatting and drive capacity: