Why is my hard drive displaying a slightly less than expected capacity ?

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Question:
Why is my drive displaying a slightly less than expected capacity in Windows or Mac?

Answer:
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.

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.

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Various Drive Sizes and their Binary and Decimal Capacities

Drive Size in GB Approximate Total Bytes

Decimal Capacity ………………………………Approximate Binary Capacity
(bytes/1,000,000,000)……………………………….(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
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
250 GB = 250,000,000,000 ………………………….250 GB = 232.83 GB

For Mac only:

For full details about this issue in Mac systems, visit Apple. The two articles from Apple listed below, explain drive translation and why the drives appear smaller:

Apple Article ID 30065
Apple Article ID 11148

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