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Why are hard drives never as large as advertised?

I just discovered a minor curiosity in My Computer.
My Computer shows that my HDD is partitioned into 3 drives:

  • C is 238 GB (Operating system)
  • D is 332 GB (Data)
  • R is 24.9 GB (Recovery)

As you can see, these partitions only add up to 594.9 GB. However, my computer was advertised as having a 640 GB HDD. Furthermore, in the System Information menu, under Components/Storage/Disks, is lists the disk size as 596.17 GB.

Does anyone understand why this is? Any help is much appreciated. Thanks!

I'm using Windows 7 64-Bit

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marked as duplicate by Karan, Simon Sheehan, ChrisF, Mokubai, Dennis Dec 31 '12 at 1:12

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1 Answer 1

It is called marketing :)

HD manufacturers measure 1GB as 1000MB and Microsoft measures 1GB as 1024MB. Hence the reason you are seeing a 640GB drive as 596GB.

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Sort of. Actually, 1 GB is 1000000000 bytes, while 1 GiB is 1073741824 bytes. Microsoft is really at fault here, labeling GiBs as GBs. Linux (that includes Mac OS), uses the proper prefixes. –  Dennis Dec 30 '12 at 3:36
The practice predates the GiB notation, so you can't blame Microsoft for not predicting the future. –  David Schwartz Jul 6 at 21:11
@DavidSchwartz yes, I can. With only a few exceptions, hard drives have always been sized and labeled using "decimal prefixes". This goes all the way back to the IBM 350 RAMAC ("5 million characters", meaning 5,000,000) and continuing through e.g. the ST506 and ST412 (5 and 10 MB, resp.) to today's multi-TB drives. When MS first started using "KB" and "MB" in reporting hard drive capacity and usage they should have followed what the hard drive makers were already doing, i.e., using decimal prefixes. See en.wikipedia.org/wiki/Binary_prefix#Disk_drives –  Jamie Hanrahan Jul 11 at 5:55
@JamieHanrahan That would have been an awful decision, making the measurements much harder to use. Back when devices are small, it was important to know how many allocation units would fit on a device. Reporting sizes in 1,000 byte units would have made that much more difficult. It was also common at the time to do real swapping (not paging). Having your RAM and disk measured in units that weren't easily comparable would have been pretty awful. –  David Schwartz Jul 11 at 15:48
There is that. However there was already a disconnect in that the "floppy megabyte" was not the same as the "RAM megabyte". –  Jamie Hanrahan Jul 11 at 19:27

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