Let me try to set the wtra fuel record straight here. Some of you used parameters and data very close to reality in their posts, some others have been trying to draw conclusions with source info miles away from real life.
1. I agree with those who say the correct parameter to be used is kg of fuel used per ton (of fuel) transported during one hour, or =kg/ton/hr. Which btw produces linear results for any flight duration, or at least quasi linear results given the higher fuel consumption at take off.
2. I agree that for a 737-300 series the consumption number is close to 40 kg/ton/hr. It may range in the 42-44 kg area for A300B4 or DC10-30 aircraft, and in the mid thirties for the most fuel efficient engines as installed on A340 etc.
But 27kg/ton/hr for a 757 has to be incorrect; that should also be close to 40kg.
3. The extra fuel, over and above the trip fuel (which already has a 5% reserve), one has to carry for safety purposes (holding, diversion, etc.)is normally known fm the flight planning data, with the ultimate decision up to the captain.
4. Any extra fuel to be taken on top of the preceding is a mgmt decision based on the company fuel policy (which would typically provide for bunkering at cheaper airports to avoid or minimize refueling at more expensive airports). If one accepts that 40 kg/ton/hr is a good average for jet aircraft, it means the cost of bunkered fuel increases by 4% (=40/1000) per flight hour. Very easy to compare your cost of bunkered fuel with the fuel cost at destination.
Hope this helps.