small b = bits
big B = bytes
if you're counting in bytes ... kilo = 1024, mega = 1024 * 1024
if you're counting bits per second kilo = 1000, mega = 1000 * 1000
This wonderful discrepancy that confuses the hell out of so many people is due to the fact that bits per second (bps) is a measure that comes from the communications industry where kilo & mega are the standard mathematical 10^3 and 10^6 values whereas bytes comes from the computer industry where kilo and mega are the values of 2^10 and 2^20 presumably chosen because they are the nearest powers of 2 to the mathematical Kilo and Mega!
so to convert say 8 Mbps (8 megabits per second) to KBps (Kilobytes/second)
8 Mbps = 1 Million Bytes per second (8 bits/byte)
1 Million Bytes /sec = 976.5625 KBps (0.95347 MBps)