Why Does my HDD Have Less Capacity than Advertised?
In truth, they don't, but this is one of those marketing ploys that are tricking you into thinking you are buying a higher capacity than you actually are.
Hard drive manufacturers market drives in terms of their decimal (base 10) capacity. In decimal notation, one megabyte (MB) is equal to 1,000,000 bytes, one gigabyte (GB) is equal to 1,000,000,000 bytes, and one terabyte (TB) is equal to 1,000,000,000,000 bytes.
All computers however use the binary (base 2) numbering system. In the binary numbering system, one megabyte is equal to 1,048,576 bytes, one gigabyte is equal to 1,073,741,824 bytes, and one terabyte is equal to 1,099,511,627,776 bytes.
Decimal and binary translate to the same amount of storage capacity, it's just advertised in a different unit.
As an illustrative mark, let's say you went running today and put up a post on Facebook. Would you put up the fact that you ran 1 kilometre or 0.621 miles? It is the same distance, but different units of measure. 1 kilometre sounds much better than 0.621 miles. The same is true for hard drive capacity.
Capacity Calculation Formula
Decimal capacity / 1,048,576 = Binary MB capacity
Decimal capacity / 1,073,741,824 = Binary GB capacity
Decimal capacity / 1,099,511,627,776 = Binary TB capacity
A 500 GB hard drive is approximately 500,000,000,000 bytes (500 x 1,000,000,000).
500,000,000,000 / 1,048,576 = 476,837 megabytes (MB) = 465 gigabytes (GB)
There is a growing call for manufacturers to list their drive capacity using binary capacity as that is what we see in Windows and what we are accustomed to using. It is often called the usable capacity of the drive.
In the table below are examples of approximate numbers that the drive may report.
|Decimal||MB (Binary)||Reported (Binary)|
|20 GB||19,073 MB||18.6 GB|
|40 GB||38,610 MB||37.3 GB|
|60 GB||57,220 MB||55.8 GB|
|80 GB||76,293 MB||74.5 GB|
|120 GB||114,440 MB||111.7 GB|
|160 GB||152,587 MB||149 GB|
|250 GB||238,418 MB||232 GB|
|320 GB||305,175 MB||298 GB|
|400 GB||381,469 MB||372 GB|
|500 GB||476,837 MB||465 GB|
|640 GB||610,351 MB||596 GB|
|750 GB||715,255 MB||698 GB|
|1 TB (1000 GB)||953,674 MB||931 GB|
|1.5 TB (1500 GB)||1,430,511 MB||1,396 GB|
|2 TB (2000 GB)||1,907,348 MB||1,862 GB|
|3 TB (3000 GB)||2,861,022 MB||2,793 GB|