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Why Does my HDD Have Less Capacity than Advertised?

By , Wednesday 28th September 2011 in Hardware

In truth, they don't, but this is one of those marketing ploys 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


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.

DecimalMB (Binary)Reported (Binary)
20 GB19,073 MB18.6 GB
40 GB38,610 MB37.3 GB
60 GB57,220 MB55.8 GB
80 GB76,293 MB74.5 GB
120 GB114,440 MB111.7 GB
160 GB152,587 MB149 GB
250 GB238,418 MB232 GB
320 GB305,175 MB298 GB
400 GB381,469 MB372 GB
500 GB476,837 MB465 GB
640 GB610,351 MB596 GB
750 GB715,255 MB698 GB
1 TB (1000 GB)953,674 MB931 GB
1.5 TB (1500 GB)1,430,511 MB1,396 GB
2 TB (2000 GB)1,907,348 MB1,862 GB
3 TB (3000 GB)2,861,022 MB2,793 GB

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About the Author

Tim Trott

Tim is a professional software engineer, designer, photographer and astronomer from the United Kingdom. You can follow him on Twitter to get the latest updates.

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