If your talking about part 121 then FAR121.639 would govern.
Every operator will have their own FAA approved FOM or "procedures manual". In this manual will be a section on fuel planning. A typical fuel planning section will say something like this.
Fuel Requirements.
No person may take off an airplane unless it has enough fuel:
To fly to the airport to which it is dispatched.
Thereafter, fly to the most distance alt. airport.
Thereafter, fly for 45 minutes at normal cruising fuel consumption.
Fuel required for dispatch must consider the following:
Wind and weather conditions forecast.
Traffic delays.
One instrument approach and missed approach at the destination.
The dispatcher determines the minimum fuel.
Contingency fuel is determined by the dispatcher.
Clear as mud right?