**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.**

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