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.

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

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