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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
(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
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