Hard Drive Capacity – Advertised vs Actual
Well recently I was asked the question “Why, when i buy an 80GB drive, does it only see around 74GB in the OS? Thats like 6GB lost!”.
Simply put, It’s base10 vs base2; or as a colleague of mine would say, “we arent comparing apples with apples”. There are 2 different methods used to calculate how much information can be stored on the hard disk drive. They are base-2 system and the base-10 system.
In the base-2 system used by the OS, 1GB = 1,073,741,824 bytes. While the base-10 system used by the manufacturing company, 1GB = 1,000,000,000 bytes. Here we begin to see the “1GB vs 1GB” discrepancy already. Now amplify it, and you begin to understand where your 6GB is 
Here is a good reference table I found on the internet a while back:
Base 2 Base 10
————————–
80 GB 76.3 GB
120 GB 114.4 GB
160 GB 152.6 GB
250 GB 238.4 GB
1000 GB 953.7 GB
Hope this helps clarify things!
Sam