Calculate Corrected Usable Hard Drive Size

Over the years there has been a  constant battle between storage engineers and  hard drive companies on whether or not advertised hard drive sizes  are misleading to consumers.  The advertised numbers are in fact misleading because they do not state the actual usable size of the hard drive being sold.  In order to find the corrected size of a hard drive you must account for the actual allocation size used by the hard drive manufacturer.

When a 300GB hard drive is sold the manufacturer is stating that the hard drive has a capacity which meets the following equation:

300GB = 300,000,000,000 bytes

With a 300GB hard drive the manufacturer would like you to think the above equation is true.  It in fact is not.  The reason why this value is false is because 1,000 bytes should be represented as 1,024 bytes.  So in order to find the true size of a  300GB hard drive you  must divide by 1024 three times to calculate its true size in Gigabytes.

300,000,000,000 bytes ÷ 1024 = 292,968,750 Kilobytes

292,968,750 kilobytes ÷ 1024 = 286,102.3 Megabytes

286,102.3 Megbytes ÷ 1024 = 279.3968 Gigabytes

The corrected size of an advertised 300GB drive is actually 279GB and change.

If hard drive manufacturers gave you the advertised capacity of 300GB the drive would have to be 322,122,547,200 Bytes in size. This can be calculated by multiplying 300GB x 1024 three times to find the required bytes.


5 thoughts on “Calculate Corrected Usable Hard Drive Size

  1. Kinda unfair assessment here.
    First, the manufacturers use SI units.
    Second, your assumption for “usable” depends on your interpretation. Usable space is actually less what you calculated depending on which filesystem is laid down on the initialized disk. To get around this unknown factor for reporting capacity, we can just state raw capacity .. which is what’s done.
    Third, you seem to misrepresent units here (SI strandard vs IEC standard). Your calculations simply show that:

    300 Gigabytes (GB) = 300,000,000,000 bytes
    300,000,000,000 bytes ÷ 1024 = 292,968,750 kibibytes
    292,968,750 kibibytes ÷ 1024 = 286,102.3 mebibytes
    286,102.3 mebibytes ÷ 1024 = 279.3968 gibibyte
    300GB = ~279.4 GiB

    Now _assume_ a filesystem, then you can produce a usable capacity, not simply dividing by 1024.

    Sorry, just setting record straight that 279.4 GiB is not end-user usable capacity.

  2. Pingback: External hard drive format - Tech Support Forums -

  3. No matter what is stated, if one states 300gb hard drive, then that is what it should have regardless of the math used , i.e.1024 is the common denominator used so forget using the 1000 as stated being the common number used . This is used by the manufactures to create smoke in order to sell their wares.

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