Calculating Fuel Consumption for Passenger Aircraft

Hi All,
I am trying to design a webpage that will calculate fuel consumption
for common passenger aircraft (mostly Boeing and Airbus models to
start). My guess is that the key variables would be distance traveled,
aircraft type, and (as a proxy for weight), percent full. I know other
variables would relate to actual flight path, holding patterns, etc.
Is there data and formulas out there I can use to calculate this? I
would greatly appreciate any pointers people can offer. I'm sure it
is possible as pilots must do it all the time. Thanks in advance.

Robin

One of the most important factor is the cruising altitude (and the air
temperature at that altitude if it's not standard temp).