PHL hit the high points. A computer does the calculations after the dispatcher puts in the flight info (from, to, alternate, routing). US used a set amount of fuel for the 45 minute reserve which depended on the aircraft type. The dispatcher would add fuel for known or anticipated delays. The 737-300/400 was the most state-of-the-art plane I flew - once the route and winds were entered after T/O it'd give arrival fuel on board, continuously updated. For holding you put in the "bingo" fuel - the fuel quantity at which a diversion was necessary - and it'd tell you how long you could hold, also continuously updated.

With the flying I did - the 737 didn't go west of MCI - contact with the dispatcher enroute was uncommon. Changes for the worse at the destination, advice on deviating for weather, that type of thing.

If holding, there was usually more back and forth with the dispatcher. Not much about the destination if it was a weather issue, but whatever else we needed or wanted info on - closer diversion options than the filed alternate, etc. We also notified the dispatcher both entering holding (along with how long we could hold) and leaving holding. And ATC was usually good about passing on any info they had that would affect how long we held.

Enroute, dispatch was always just a message away. We could request the fuel burn for alternate altitudes or routing, etc.

Then there's those times that all that planning goes out the window. One night we were flying PHL-GSO and there was a big weather system in southern VA and the Carolinas. ATC routed us over CVG, eastern TN and finally into GSO from the southwest, so we had fuel for that routing. On taxi-out, ATC cancelled that routing and gave us the standard almost straight line routing so we suddenly had a bunch of extra fuel. So much extra that we would be about 5,000# over max landing weight when we arrived at GSO.

Jim