A common marketing ploy by hard drive manufacturers is to calculate the size of a hard disk by using the decimal 10 system of 1000 bytes = one kilobyte, instead of the binary system  where 1024 bytes = one kilobyte (your PC only knows binary). This rounding off practice means you end up with a hard drive with a capacity  less than what is indicated on the label. For example, a true 80 GB hard disk can hold  85,899,345,920 bytes, but by using the base 10 formula,  80 GB comes out to only 80,000,000,000 bytes  Do the simple subtraction, that's  5,899,345,920 bytes less than the true value.  Now calculate what 5,899,345,920 bytes converts to by entering 5899345920 in the Byte box above. You will find that it translates to a  loss of 5.49419 GIGABYTES!

To perform the calculation manually:

True Size:   80 x 1024 X 1024 x 1024 = 85,899,345,920 bytes
False Size:  80 x 1000 x 1000 x 1000= 80,000,000,000 bytes

And there you have the reason for the size discrepancy when you correctly  install a hard drive in Windows and its Properties report it to be smaller than what you thought. A PC only understands binary, 0's and 1's, and that's how Windows calculates your hard drive size. This means that an  80 GB drive, figured using the decimal 10 system, would only show as about 74.5GBs  in Windows.  A 40 GB drive would only show as about 37.2 GB  The difference works out to about 7% less.

If you want a simple way to confirm this for yourself, I'm going to give you something to try. But before starting, you must understand that the definition of ONE BYTE is 8 bits which equals a single plain text unformatted character.  Anytime you press a character with your keyboard you are using one byte. So, with that in mind, let's first begin by confirming that fact. We want to confirm that when we add a byte, Windows will see it as a byte and then we can see just how many more bytes we need to add to make a kilobyte.

Open up a new notepad document and  type the letter A (or any other letter).  Just a single character.
Save the file, and then right click it and go to Properties to see the Size. It should indicate one byte (the size indicated in brackets is always the the true size in bytes) like this:

Ignore the "bytes used" size - it doesn't pertain to this test - the bytes used just refers to the cluster size - i.e., how much space the file actually uses up on your hard drive.

Now close it,  delete it, and open up another new text document, full screen, with Word Wrap On.  Enter 1,024  continuous characters to give you 1024 bytes.   Note - every key press takes up one byte of memory so if you hit any other key like Enter, CTR or Shift, you will be adding bytes and your count will be inaccurate.

To make it easier, below  are 100 characters.

1234567890123456789012345678901234567890123456789012345
678901234567890123456789012345678901234567890

First, copy and paste them into your new text document, save it,  close it,  and look at the Properties to make sure you pasted correctly. File size should read 100 bytes.  Make sure line breaks here, do not put spaces between the numbers. Then reopen it, and continue pasting the 100 character line 9 more times without pressing any other keys or leaving any spaces, so that you will have exactly 1000 bytes.  Then type in 24 more characters to bring the total up to 1024 bytes.

Once you've entered your 1024 characters, save and go to Properties again to look at the size. It should read like this:

As you can see,  1,024 bytes is equal to exactly 1.00 KB

Now open up your 1.00KB text document, highlight  24 characters and delete them to make it 1000 characters. Save it and look at the size again in Properties:

Now you see that 1000 bytes is only .97KB and this illustrates how using the decimal system to calculate a kilobyte is inaccurate.

The 1000 bytes you used does not translate to a full kilobyte.

If you were to market that 1000 byte file as 1 KB, your customers would be getting ripped off by 24 bytes on every 1 KB purchase.

I really don't know which hard drive makers - if any - use the the binary system to calculate hard drive sizes, but recently a lawsuit was launched against Dell, Gateway, Apple, HP, and others over this matter.  See here for the story:

http://www.wired.com/news/business/0,1367,60505,00.html

Bits and Bytes for Newbies:

BIT - the smallest unit of information a computer can use, either a 1 or 0, and all it knows how to use!

BYTE - consists of 8 bits which patterns one character which can be a letter, number or symbol  For example, 01000001 = A

Kilobyte  - consists of 1024 bytes which equals 1024 characters which equals about a page of double spaced text.

Megabyte - consists of 1,048,576 bytes which is enough bytes to hold a book of maybe 500 pages or more or a 50 page book if it contains graphics.   A floppy disk can hold 1.44 Megabytes.

Gigabyte - consists of  1,073,741,824 bytes which is enough bytes to hold a complete set of encyclopedia including graphics, audio and video.

Terabyte - consists of one trillion bytes (1000 Gigabytes)

And if you think no one could ever need  terabytes of  storage, then think again.  Less than 4 hours or so of digital video can eat up a whole terabyte.  Over 4.5 GB per minute is needed to achieve the DV quality standard which is 720x480 resolution and a 5 to 1 compression.

NOTE: If you are new to all this and would like to learn more about bits and binary, look for my article  "Just A Little Bit" in my TechTrax archives.
http://pubs.logicalexpressions.com/Pub0009/LPMFrame.asp?CMD=ArticleSearch&AUTH=7