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Why is the effective hard drive size lower than the actual size?

If one buys a 500 GB hard disk, why it shows only 465 GB. Where does the extra space go? Same for other secondary storage devices like pen drives. What is the reason?

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migrated from stackoverflow.com Nov 27 '12 at 14:21

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marked as duplicate by Oliver Salzburg Nov 27 '12 at 14:51

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2 Answers 2

1 kilobyte is actually 1024 bytes. So

500,000,000,000 / (1024*1024*1024) = 465.66 GB

from howtogeek.com:

To a hard disk manufacturer, one KB is 1000 bytes, one MB is 1000 KB, and one GB is 1000 MB. Essentially, if a hard disk is advertised as 500GB, it contains 500 * 1000 * 1000 * 1000 = 500,000,000,000 bytes of space.

However, manufacturers of RAM don’t sell it in even groups of 1000 – they use groups of 1024. When you’re buying memory, a KB is 1024 bytes, a MB is 1024 KB, and a GB is 1024 MB.

Unfortunately, Windows has always calculated hard drives as powers of 1024 while hard drive manufacturers use powers of 1000.

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500 Marketing GByte (1000*1000*1000 bytes) = 465 Engineering GByte (1024*1024*1024) :-) –  marc_s Nov 27 '12 at 14:26
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See also KibiByte –  MDeSchaepmeester Nov 27 '12 at 14:35

That's because 500 GB (for sellers) means 500.000.000.000 bytes, which are not exactly 500* (1024*1024*1024) = 500 * 1073741824, where 1073741824 is the number of bytes of 1 GB.

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In addition, some of the storage space goes in the format of the drive. MBR, file allocation tables, etc. –  Lee Taylor Nov 27 '12 at 18:19