Technology blog::Life hacks::Linux::Hardware::Gaming

Why Does my HDD Have Less Capacity than Advertised?

By Tim Trott, 28th September 2011 in Hardware

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.

Computer Hard Drives
Computer Hard Drives

Decimal and binary translates 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 of 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 = Decimal TB capacity

Example

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)

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
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