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Determining drive capacity can be confusing at times because of the different measurement standards that are often used. When dealing with Windows and Mac based systems, you will commonly see both decimal measurements and binary measurements of a drive's capacity. In either case, a drive's capacity is measured by using the total number of bytes available on the drive. As long as the drive displays the correct number of bytes (approximate), you are getting the drive's full capacity.
Note: For more information about this issue in Mac operating systems, visit Apple. There are two articles that Apple uses to explain why the drives appear smaller than adverstised: Apple Article ID 30065 and Apple Article ID 11148.
Decimal vs. Binary:
For simplicity and consistency, hard drive manufacturers define a megabyte as 1,000,000 bytes and a gigabyte as 1,000,000,000 bytes. This is a decimal (base 10) measurement and is the industry standard. However, certain system BIOSs, FDISK and Windows define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. Mac systems also use these values. These are binary (base 2) measurements.
To Determine Decimal Capacity:
A decimal capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,000,000,000 using base 10).
To Determine Binary Capacity:
A binary capacity is determined by dividing the total number of bytes, by the number of bytes per gigabyte (1,073,741,824 using base 2). This is why different utilities will report different capacities for the same drive. The number of bytes is the same, but a different number of bytes is used to make a megabyte and a gigabyte. This is similar to the difference between 0 degrees Celsius and 32 degrees Fahrenheit. It is the same temperature, but will be reported differently depending on the scale you are using.
Important: If you are dealing with drive sizes smaller than the Approximate Binary Capacity of your Drive Size, then you may be dealing with either a BIOS limitation, or a Windows drive size limitation. For more information, please see links below.
Various Drive Sizes and their Binary and Decimal Capacities
| Drive Size |
Approximate Total Bytes |
Decimal Capacity
(bytes/1,000,000,000)
|
Approximate Binary Capacity (bytes/1,073,724,841) |
| 10 GB |
10,000,000,000 |
10 GB |
9.31 GB |
| 20 GB |
20,000,000,000 |
20 GB |
18.63 GB |
| 30 GB |
30,000,000,000 |
30 GB |
27.94 GB |
| 36 GB |
36,000,000,000 |
36 GB |
33.53 GB |
| 40 GB |
40,000,000,000 |
40 GB |
37.25 GB |
| 60 GB |
60,000,000,000 |
60 GB |
55.88 GB |
| 74 GB |
74,000,000,000 |
74 GB |
68.91 GB |
| 80 GB |
80,000,000,000 |
80 GB |
74.51 GB |
| 100 GB |
100,000,000,000 |
100 GB |
93.13 GB |
| 120 GB |
120,000,000,000 |
120 GB |
111.76 GB |
| 150 GB |
150,000,000,000 |
150 GB |
139.69 GB |
| 160 GB |
160,000,000,000 |
160 GB |
149.01 GB |
| 180 GB |
180,000,000,000 |
180 GB |
167.64 GB |
| 200 GB |
200,000,000,000 |
200 GB |
186.26 GB |
| 250 GB |
250,000,000,000 |
250 GB |
232.83 GB |
| 300 GB |
300,000,000,000 |
300 GB |
279.40 GB |
| 320 GB |
320,000,000,000 |
320 GB |
298.02 GB |
| 400 GB |
400,000,000,000 |
400 GB |
372.52 GB |
| 500 GB |
500,000,000,000 |
500 GB |
465.65 GB |
| 640 GB |
640,000,000,000 |
640 GB |
595.84 GB |
| 750 GB |
750,000,000,000 |
750 GB |
698.47 GB |
| 1 TB (1000 GB) |
1,000,000,000,000 |
1 TB (1000 GB) |
931.30 GB |
| 1.5 TB (1500 GB) |
1,500,000,000,000 |
1.5 TB (1500 GB) |
1396.95 GB |
| 2 TB (2000 GB) |
2,000,000,000,000 |
2 TB (2000 GB) |
1862.6 GB |
| 3 TB (3000 GB) |
3,000,000,000,000 |
3 TB (3000 GB) |
2793.97 GB |
| 4 TB (4000 GB) |
4,000,000,000,000 |
4 TB (4000 GB) |
3725.29 GB |
| 6 TB (6000 GB) |
6,000,000,000,000 |
6 TB (6000 GB) |
5587.94 GB |
| 8 TB (8000 GB) |
8,000,000,000,000 |
8 TB (8000 GB) |
7450.58 GB |
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