It's all about Decimal vs. Binary measurements...
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.
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).
However, certain system BIOSs, FDISK and Windows define a megabyte as 1,048,576 bytes and a gigabyte as 1,073,741,824 bytes. These are binary (base 2) measurements.
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.
For instance...
Various Drive Sizes and their Binary and Decimal Capacities
Drive Size in GB Approximate Total Bytes Decimal Capacity | Approximate Binary Capacity (bytes/1,073,724,841)
(bytes/1,000,000,000)
10 GB ----------------------------------------------------------------------------9.31 GB
20 GB -------------------------------------------------------------------------- 18.63 GB
30 GB -------------------------------------------------------------------------- 27.94 GB
40 GB -------------------------------------------------------------------------- 37.25 GB
60 GB -------------------------------------------------------------------------- 55.88 GB
80 GB -------------------------------------------------------------------------- 74.51 GB
100 GB ------------------------------------------------------------------------ 93.13 GB
120 GB ----------------------------------------------------------------------- 111.76 GB
160 GB ---------------------------------------------------------------------- 149.01 GB
180 GB ----------------------------------------------------------------------- 167.64 GB
200 GB ----------------------------------------------------------------------- 186.26 GB
250 GB ----------------------------------------------------------------------- 232.83 GB
