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What is the actual capacity of a 1TB HDD, is it 1000GB or 931GB?

I am not asking about the formatted size; the raw/unformatted size. The data sheet says, 1TB HDD is 1 Trillion Bytes, but if you check Google with "one trillion bytes in GB" it says, 931.323GB.

At the same time, Wikipedia says, 1TB is 1000GB and it is the 1TiB as 931GB.

Now I'm a bit confused, actually a 1TB HDD is manufactured as 1024GB or 1000GB or 931GB? or is the 1000GB in SI same as 931GB in binary?

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The "unformatted" capacity of a HDD is irrelevant today, as the disk controller does not allow access to the unformatted drive (anymore). Note that this "formatted" capacity (whether in decimal or binary multipliers) is further reduced by disk partitioning and filesystem overhead. Installing a filesystem is confusing also called "formatting", which is not the low-level media format when HDD manufacturers mention "unformatted" capacity. –  sawdust May 24 '13 at 20:05

2 Answers 2

up vote 2 down vote accepted

You're thinking in the right direction. The "problem" rises from trying to express binary sizes in base 10 numbers which consumers are more familiar with.

2 links which explain the problem in more detail:

http://compreviews.about.com/od/storage/a/ActualHDSizes.htm http://www.techzonez.com/forums/showthread.php/7243-Drive-Capacity-explained

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great!, So to conclude, a 1TB HDD is 1000GB as advertised but is only 931GB if you are thinking in terms of 1KB=1024Bytes... –  Aczire May 24 '13 at 8:28
Exactly. In the past, there have been some disputes about this, especially concerning false advertisement. There's a collection of some on Wikipedia: en.wikipedia.org/wiki/Binary_prefix#Legal_disputes –  SBI May 24 '13 at 8:34

Manufacturers like to express the size of their hard drives in base-10 numbers, then call this mega/giga/tera. This is factually incorrect, but the drives look bigger.

Example: They make a 1000 GB drive and call it 1 TB
Formally, since mega/giga/tera should be base-2 numbers, 1024 GB is 1 TB, so it's not a real 1 TB drive.

But if you now go all the way down to bytes, it gets worse ;-)

Their gigabytes are not real 1024 MB, but actually 1000 MB.
Their megabytes are not real 1024 kB, but actually 1000 kB.
Their kilobytes are not real 1024 bytes, but actually 1000 bytes.

So: make a disk of 1000 000 000 000 bytes, call it 1 TB, and strictly speaking you have 1000 000 000 000 / 1024 / 1024 /1024 = 931 GB

[Reminds me of the "60 MB" backup tapes we used to buy, which were actually 30MB tapes, and the manufacturer assumed a compression ratio of 2]

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