Why does my hard drive report less capacity than indicated on the drive's label?


Explains why a hard drive's capacity often appears to be less than advertised when showing in (My) Computer or Mac Disk Utility.

Hard drive manufacturers market drives in terms of 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.

Programs such as FDISK, system BIOS, Windows, and MacOS 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.

Simply put, decimal and binary translates to the same amount of storage capacity. Let's say you wanted to measure the distance from point A to point B. The distance from A to B is 1 kilometer or .621 miles. It is the same distance, but it is reported differently due to the measurement.

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) Windows Output (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

Below is an example of a 3TB drive displayed in Windows.

Image

Please also see Document ID: 194563 and the National Institute of Standards and Technology at http://physics.nist.gov/cuu/Units/binary.html





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