fanout is the number of gates that can be connected to a single output pin of an ic, so that it can drive those gates (with the required current ratings) ..condition is that those gates must be of the same logic family..
you can experiment it out by calculating the fan out experimentally and then connecting different gates of other logic families..the results will vary..
ex- if you connected some other gates to a nand gate output pin of "nand gate ic", you will get the fanout as specified in the ic's datasheet provided as by the current ratings as :
Iol(low level output current), Iil(low level input current), Ioh(high level output current), Iih(high level input current),,
keeping these in mind,, you can now understand fanout as this:
fanout(low)-the number of gates that a output pin can drive when the pin is at low state..
looking at the datasheet of 7400 ic..
i got Iol as 8mA and Iil as 0.4mA, Ioh as 400uA, Iih as 20uA...(you may confirm to the datasheet also for the values)
so fanout(low)=Iol/Iil
=20
fanout(high)-outputs from a single pin that can drive other gates while it is in high state..
=Ioh/Iih
=400/20=20
it is just a case that both are coming out to be the same,, otherwise if they are different,, lower value is used..
You can yourself verify these out...
for deeper studies:you may refer to some good books like of Digital Systems, Principles and application by Tocci and Moss... etc.....