Hard Drive Space Discrepancy

You may notice that your storage device does not seem to have exactly the amount of free space that it is advertised with. In reality the space is still there. The difference in reported space is down to a complex difference between decimal counting versus binary counting.

Drive manufacturers often advertise their storage space using decimal gigabytes, where the definition of a gigabyte is 1,000,000,000 bytes. This is the size you will find on the packaging and label and is also the figure used when advertising products online. Using this method of calculation (1 GB = 1000000000 bytes (= 10003 B = 109 B)), a manufacturer will be able to label a storage device as being a convenient, marketable “500gb Hard Drive” (for example).

This difference in space is due to an alternate calculation which is carried out by the operating system (Windows in this case), which uses binary rather than decimal. In binary, a gigabyte is defined as 1,073,741,824 bytes. In practice, this calculation (1 GiB = 1073741824 bytes (= 10243 B = 230 B)) will result in a “500gb Hard Drive” showing as having 465gb of “actual space”. This difference is exponential, resulting in a greater difference the larger the storage device becomes.

This is an industry standard and does not indicate a fault with your storage device. ALL drives will show the discrepancy under most operating systems, regardless of make or model.

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This is a good explanation of why the advertised size and actual storage space on a HDD differ

http://tierradatarecovery.co.uk/why-does-my-1tb-hard-drive-offer-only-931gb-of-space

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In fairness, apart from the baffling mathematics, most descriptions of this discrepancy do quite well explaining it. Simply boils down to 2 different calculations, a marketing one and a actual one.