Here is the old fashion and basic way to calculate fuel.

First we need to calculate our true air speed which is called KTAS (Knots True Airspeed) and each plain is different.  

Example:
280 KIAS  (757 or 767) at FL360

280 x 2% (0.02)= 5.6
5.6 x 36 (FL360)= 202
202 + 280= 482 is your KTAS.  Remember, each air craft performs different and at different flight levels, but this is the basic formula.

We have discovered that at 280 KIAS means that we are traveling at 482 KTAS.  How many miles can we cover at 482 KTAS?

482 KTAS / 60 minutes= 8.0 NM . . . we can cover 8 miles at 482 KTAS every minute.  Our example flight will be from KATL (Atlanta) - KLAS (Las Vegas)

KATL - KLAS is 1559 NM

1559 NM / 8.0 NM per minute your traveling= 194.87 + 10 (extra)= 205 minutes by rounding up . . . . this flight will take 205 minutes, but lets divide by hours.

205 minutes / 60= 3.4 hours of flight time.  Know lets figure out our Alternate Airport which is KLAX (Los Angeles).

KLAS - KLAX is 205 NM
205 NM / 8.0 NM= 26 minutes
26 minutes / 60= 0.4

     This is really important!  

 We must find out how much PPH (pounds per hour) or FF (fuel flow) is used per engine.  On a 757 it can be about 6,500 lbs to just over 7,000 lbs per engine.  The best way to figure your PPH or FF is take the 757 or 767 . . . for a flight.  Once airborne and level at a average flight level mark the qty of fuel you have on board through your "fuel and payload tab" and fly for one hour and then mark the qty you have used and subtract.  This will be your fuel used for both engines per hour.

 For this example, we are going with the 757 and we will calculate fuel for one engine alown, so what ever your test results was, divide it in half and it should be roughly 6,800 per engine.

Flight time
3.4 x 6,800=                23,400 lbs per engine.

Reserves (45 minutes worth)
45 / 60= .75
.75 x 6,800=                  5,100 lbs per engine  

Hold Fuel (30 minutes)
30 / 60= 0.50
.50 x 6,800=                  3,400 lbs per engine

Taxi (30 minutes) The PPH or FF is nothing compare to FL360 feet, so we will use about 1,950 lbs per engine
30 / 60= 0.50
.50 x 1,950=                     975 lbs per engine

Total need to fly from KATL - KLAS

Flight level ---- 23,400
Reserves ------  5,100
Hold ------------  3,400
Taxi ------------     975

Total ----------- 32,875 lbs per engine.                

This is all per engine and the 757 has two engines, so we multiply this total

32,875 lbs x 2= 65,750 PPH or FF . . . . . So you would need to load 65,750 lbs of fuel.  I hope this helps