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From all the hard drives I have bought, they never seem to be as large as the advertised price; from 320GB down to 290GB, from 500GB down to 450GB, etc. Is there a technical reason for this?

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The nontechnical reason, of course, is that the manufacturers will put as large a number as they can possibly justify on the box, to drive sales. It's similar to ads with small print "up to"s on the capabilities and "starting at"s for the prices. –  David Thornley Dec 10 '09 at 15:13
Your drives are as large as advertised. The operating system just measures them wrong. –  endolith Sep 4 '10 at 20:03
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6 Answers

up vote 17 down vote accepted

The technical reason is that the hard drive manufacturers sell you capacities in metric units. So a GB = 1,000,000,000 bytes by the metric system. However, computers measure the drive size in powers of 2. So 1GiB = 1,024MiB, 1MiB = 1,024KiB, etc. What this means is that 1GiB = 1,073,741,824 bytes, a difference of 73,741,824.

So when you install your 1GB (for the sake of example) drive, the OS only sees 0.93GiB, and this is the cause of the discrepancy.

(If you've never seen the abbreviation GiB before, it's a new notation adopted to denote powers of 1024 as opposed to 1000. However, most operating systems will report GiB as GB, confusing this issue even further)

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Gi => Gibi rather than G => Giga –  ChrisF Jul 15 '09 at 9:12
@ChrisF: yep, I added an addendum to my post explaining that –  Kyle Cronin Jul 15 '09 at 9:13
And don't get me started on the old "1.44Mb" floppy disks. These were actually made out of 1440 * 1024 bytes, using both the 1000 and 1024 measure simultaneously. It wasn't neither MiB nor MB –  R. Martinho Fernandes Jul 15 '09 at 9:35
Wikipedia has a writeup and chart showing the differences en.wikipedia.org/wiki/Hard_disk_drive#Capacity_measurements –  Chris Nava Dec 10 '09 at 15:39
Apple recently changed the display of disk sizes within MacOSX to use metric values. –  Chris Nava Dec 10 '09 at 15:44
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When a drive manufacturer creates a 500 GB capacity drive, it does have a capacity of 500,000,000,000 bytes, and they are sure going to advertise it as such. Computers, being binary devices, prefer powers of two, with a different set of prefixes, so that is what they use for storage space measurement:

1 kibibyte = 2^10, 1 mebibyte = 2^20, 1 gibibyte = 2^30, etc.

For instance, I have a 300 GB drive attached to this machine and Windows displays the following for the capacity:

Capacity:          300,082,855,936     279 GB

300,082,855,936 / 2^30 = ~279. What it is actually showing you is the drive's size in gibibytes, not gigabytes. So, it should read:

Capacity:          300,082,855,936     279 Gi

One might say this is a flaw in Windows, but apparently there is no definitive standard for storage capacity prefix meanings. Lots more good info, including a section on "Consumer confusion", in this Wikipedia article.

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See this article for an explanation.

Basically, there are two definitions of a "gigabyte". One definition is that 1GB = 10243 bytes. This is the definition that the computer reports (for technical reasons).

The other definition (from SI units) is that 1GB = 10003 bytes. This is the same as every other metric unit ( 1 gigameter = 10003 meters).

Since the metric definition of a gigabyte is less than what the computer considers a gigabyte, hard drive manufacturers use the metric definition because they can print a larger capacity on the box.

A small amount of space is also used by the file system itself, but most of the missing capacity is from the definition of a gigabyte.

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They actually usually are as large as they are advertised, but:

  1. They always (as far as I know) use 1000 instead of 1024 when doing B to KB and so on.
  2. Some small amount of space is used by the file system to keep track of everything.

May be other reasons too, but those are the major ones I know about

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Because in technical terms 1Gb = 1024Mb (1,073,741,824 bytes), but in marketing terms 1Gb is 1,000,000,000 bytes.


That, and you lose some space with stuff like allocation tables, boot records and stuff.

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A certain amount is used in the installation of the operating system it self, but I think as much as 50 gb is too much.

In my case it have been about 5-8 gb of difference

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Most operating systems report the capacity of the drive, regardless of space taken up by the OS itself. This is not the reason... –  user487 Jul 15 '09 at 9:17
Mmhh you're right. I was thinking on free space :) –  OscarRyz Jul 15 '09 at 9:18
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