Actually, the discrepancy lies in the mixed usage of binary and decimal kilobytes. The number system that everyone is most familiar with is the decimal, or base 10, system, where every position in a number represents a power of 10. For example, the number 123 can be broken down into 1*10**2 (1 times 10 squared, or 100) + 2*10**1 (2 times 10 to the first power, or 20) + 3*10**0 (3 times 10 to the 0th power, which is 1, or 3). In Greek, "kilo" means one thousand, and that's what everyone originally accepted it as being. A kilometer is 1000 meters, not 1024 meters.
But when computers were being developed, they (nearly) all used a binary numbering system, and the power of 2 closest to 1000 was 2**10 (2 to the 10th power), or 1024. So it was decided to use the term kilobyte to describe this value. The binary number system was never designed to count a small number of things (other than its "binary" values, 0 and 1). It was used to measure bytes, of memory, and of disk space, which even in the early days of computers were numbered in the thousands.
And for a number of years, everyone cooperated with everyone else, and all numbers agreed, because the hard drive manufacturers reported their drives' capacities in binary units: kilobytes and megabytes, while operating systems (DOS and Windows) also reported disk capactities in binary kilobytes and megabytes.
So a binary kilobyte is "almost" the same as a decimal kilobyte, being off by only 2.4%. System capacities grew, however, and the discrepancy between the two numbering systems grew as well.
Decimal Binary Discrepancy
------- ------ -----------
Kilobyte 1000 1024 2.4%
Megabyte 1000000 1048576 4.9%
Gigabyte 1000000000 1073741824 7.4%
As hard drives crept into the gigabyte size, drive manufacturers decided that the discrepancy was costing them money. A 10-gigabyte drive had almost 11 million bytes of storage, while a 15-gigabyte drive had OVER 16 million bytes of storage. So they decided to begin reporting their drives' capacities using decimal measurements.
That 80-GB hard drive that you have has 80 decimal gigabytes on it, or 80,000,000,000 bytes (and even a few more than that, most likely). Windows, however, is measuring that drive's capacity using binary gigabytes, and it sees 80,000,000,000/1,073,741,824 binary gigabytes, or 74.498867721509047872585003729562 gigabytes. (Call it 74.5 gigabytes.) If your system reports that you've got 75 gigabytes, then there's actually a few more than just 80,000,000,000 bytes there.
Sorry for the long-winded explanation. It's just the teacher in me that keeps popping up. (Down, boy!)
Rich (in Minn.)
