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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, and one Gigabyte (GB) is equal to 1,000,000,000 bytes.

Programs such as FDISK, system BIOS, and Windows use the binary (base 2) numbering system. In the binary numbering system, one megabyte is equal to 1,048,576 bytes, and one gigabyte is equal to 1,073,741,824 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 one 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

Example:
A 40 GB hard drive is approximately 40,000,000,000 bytes (40 x 1,000,000,000).

40,000,000,000 / 1,048,576 = 38,162 megabytes

In the table below are examples of approximate numbers that the drive may report.
Decimal Binary MB Windows Output
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

Below is an example of a 120GB drive displayed in Windows.