Manufacturers calculate drive capacity slightly differently than how computers actually see usable storage space. Manufacturers count drive capacity using the decimal system, while computers count drive capacity using the binary system. So, when you wish to convert to one system of measurement to another, there are discrepencies, much like the way there are discrepencies when converting between the Celcius and Fahrenheit temperature scales.
So, to calculate the actual drive capacity, take the manufacturer’s advertised capacity and divide it by 1.024 three times. For example, if you take an 80GB drive capacity and divide it by 1.024 three times, you will get 74.5GB of actual storage space.
For referece, here’s a list of common drive capacities matched with their actual data capacities (rounded to the nearest hundreths place):
10 GB = 9.31 GB
20 GB = 18.63 GB
30 GB = 27.94 GB
36 GB = 33.53 GB
40 GB = 37.25 GB
60 GB = 55.88 GB
74 GB = 68.91 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
300 GB = 279.40 GB
320 GB = 298.02 GB
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