Disk Overhead

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Disk overhead is the reason why a computer hard drive or storage device's capacity may show its capacity as different from its actual capacity.

[edit] Example

For example, a computer company sells a hard drive that is marked as 200 gigabytes. When connected to a computer, it is shown that the actual storage capacity is 181.9 GiB (for example).

[edit] Explanation

One common reason is that the computer company or hardware manufacturer reflects the size of the hard drive either in the standard SI-prefix system (where 1 kB = 1000 bytes) or the Binary-prefix system (where 1 kB = 1024 bytes). To avoid confusion, the Binary prefix symbols were renamed KiB, MiB and GiB with the advent of the IEC system. In this context, one KiB equals 1024 bytes. However, which convention the manufacturers or operating system use usually depends on them.

1 KB = 1,000 bytes = 0.98 KiB
1 MB = 1,000,000 bytes = 0.95 MiB
1 GB = 1,000,000,000 bytes = 0.93 GiB
1 TB = 1,000,000,000,000 bytes = 0.91 TiB

and conversely

1 KiB = 1024 bytes = 1.02 KB
1 MiB = 1,048,576 bytes = 1.05 KB
1 GiB = 1,073,741,824 bytes = 1.07 GB
1 TiB = 1,099,511,627,776 bytes = 1.10 TB

Finally, some space inside a hard drive can also be used by the file system for its internal use (ex: FAT table) (usually less than 1% on drives larger than 100MB), or for data integrity and fault-tolerance (like RAID) (i.e. in a RAID1 you will loose 1/2 of your space to mirroring, and in a RAID5 with x drives you will lose 1/x of your space to parity). The latter is only seen in devices that contain multiple drives but are marketed as a single drive (i.e. NAS devices and external HDDs).