Question
Why does my Promise product report a smaller hard drive capacity than I expected?
Solution
Determining drive capacity can be confusing at times because of the different measurement standards that are often used. When dealing with Windows and PC 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.
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 Explorer define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. These are binary (base 2) measurements.
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). 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. It's the same number of bytes, but different ways of counting a megabyte and 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.
Below you will find some approximations of how various drive capacities will appear:
|
Drive Size |
Approximate |
Decimal Capacity |
Approximate |
|
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 |
|
40 GB |
40,000,000,000 |
40 GB |
37.25 GB |
|
60 GB |
60,000,000,000 |
60 GB |
55.88 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 |
|
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 |
|
500 GB |
500,000,000,000 |
500 GB |
465.66 GB |
|
750 GB |
750,000,000,000 |
750 GB |
698.49 GB |
|
1000 GB (1 TB) |
1,000,000,000,000 |
1000 GB (1 TB) |
931.32 GB |
Remember, as long as the total number of bytes is correct, you are getting the full capacity of the drive.
