Thursday, April 5, 2012

Why My actual HDD space is not showing in OS ?

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
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.
Some consumers feel short-changed when they discover the difference, and claim that
manufacturers of drives and storage devices are using the decimal measurements in an
intentionally misleading way to inflate their numbers.

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 BIOS, 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.


320 GB in decimal notation = 298 GB in binary notation. Windows/Linux report drive sizes in binary, the manufacturers use decimal for marketing purposes and nice round numbers.
1 GB in decimal = 10^9 or 1 000 000 000 bytes.
1 GB in binary = 2^30 or 1 073 741 824 bytes
1000000000 / 1073741824 = ~0.931 ratio between a decimal and binary gigabyte
320 GB * 0.931 =~ 298 binary GB.