What do terabytes measure

Why is my hard drive showing less storage capacity than stated on the hard drive label?


This article explains why the capacity of hard drives on Windows or older macOS versions often seems smaller than stated.

The hard drive manufacturers market the capacity of the hard drives in decimal units. In decimal notation, a megabyte (MB) is 1,000,000 bytes, a gigabyte (GB) is 1,000,000,000 bytes, and a terabyte (TB) is 1,000,000,000,000 bytes.

Programs such as FDISK, the BIOS system, Windows, and older versions of macOS use the binary numbering system. According to the binary numbering system, a megabyte is 1,048,576 bytes, a gigabyte is 1,073,741,824 bytes and a terabyte is 1,099,511,627,776 bytes.

Formula for calculating the capacity

Decimal capacity / 1,048,576 = binary capacity in MB 
Decimal capacity / 1,073,741,824 = binary capacity in GB
Decimal capacity / 1,099,511,627,776 = binary capacity in TB

Example:
A 500 GB hard drive has a capacity of approximately 500,000,000,000 bytes (500 x 1,000,000,000).
The binary gigabyte calculation (500,000,000,000 / 1,073,741,824) shows 500 GB as 465 gigabytes.
Therefore, Windows shows a 500 GB hard drive as a 465 GB hard drive.

A 5 TB hard drive has a capacity of approximately 5,000,000,000,000 bytes (5 x 1,000,000,000,000).
The binary terabyte calculation (5,000,000,000,000 / 1,099,511,627,776) shows 5 TB as 4.54 terabytes.
Therefore, Windows shows a 5 TB hard drive as a 4.54 TB hard drive.

The table below shows examples of the approximate numerical values ‚Äč‚Äčthat may appear on the hard drive.

Capacity on the product (decimal)Mac OS X performance (decimal)Windows performance (binary)
500 GB500 GB465 GB
1 TB (1,000 GB)1 TB (1,000 GB)931 GB
2 TB (2,000 GB)2 TB (2,000 GB)1.81 TB
3 TB (3,000 GB)3 TB (3,000 GB)2.72 TB
4 TB (4,000 GB)4 TB (4,000 GB)3.63 TB
5 TB (5,000 GB)5 TB (5,000 GB)4.54 TB
6 TB (6,000 GB)6 TB (6,000 GB)5.45 TB
8 TB (8,000 GB)8 TB (8,000 GB)7.27 TB
10 TB (10,000 GB)10 TB (10,000 GB)9.09 TB

 

Below is an example of a 3TB hard drive on Windows.

Please note that the capacity of the 3 TB hard disk (3,000,000,000,000 bytes) is specified with the decimal and binary value (red or blue circle).

Put simply, the decimal and binary notation correspond to the same storage capacity. For example, suppose you want to measure the distance between point A and point B. The distance from A to B is 1 kilometer or 0.621 miles. This means that the distance is identical, but can be specified in different units of measurement.

 

For more information, see Document ID: 194563 and the National Institute of Standards and Technology at http://physics.nist.gov/cuu/Units/binary.html.