How do you convert from kb/s to kB/s?

It can be difficult to distinguish between the two terms ‘kilobits per second’ and ‘kilobytes per second’.
A byte consists of 8 bits. You should therefore start by dividing the number of kbit by 8.
Next, you need to subtract a percentage for overhead, which varies depending on the type of line.
For ADSL, this is around 10–15 per cent due to the overhead of ATM cells, which comprise at least a 5-byte header for every 48 bytes of data.

The calculation for a 256 kbit/s ADSL line would then look like this:
256 / 8 = 32 kbyte/s
32 * 88% = 28.16 kbyte/s

You must then also deduct the types of overhead that are always present on internet connections, namely IP overhead and (usually) TCP overhead.

As a rule of thumb, you can therefore expect to need to divide by 9 or 10 to work out the effective speed of a line.