How I Did It

  Officially, there are 1024 bits in a kilobit, 1024 bytes in a kilobyte, and so on. Operating systems use this method to find file size. Hardware manufactures, however, use 1000 bits are in a kilobit to measure the speed or capacity of their products. This applet uses the 1024 method to calcutate because it looks for the downoad time of the file, which is the right way. The applet display shows the rated time.

The source code shows how the the math is done in detail.

Source- whatis.com