This question has been asked time and time again. In order to answer this question you will need some reference data from your
aircraft.

You can of course make a short flight and look in the FsPassengers flight report for the fuel consumption rate per hour. (then see below for calculation)
Or bring up the in-flight report during flight (see FsPassengers flight menu) and look at the fuel data.

OR

You can search the internet about fuel consumption per hour of your aircraft type.
Example with the 737-800: Google search fuel consumption per hour 737-800

OR

You can use a fuel planner add-on: Google search Fuel planner for FS9

OR

Start by loading up your aircraft with about 50% fuel, weather set to clear skies, 59F(15C). Take off from your desired
airport and climb up to the normal cruise altitude for that aircraft. Now, look at the gauges and note your fuel rate of burn per engine.
Generally speaking, most gauges give this in pounds/hour, however, some panels may give this in kilograms/hr or even pounds/minute!
In short, look at one gauge, and then multiply that by the number of engines you have on your aircraft. You should have a rough estimate
of your fuel consumption per hour of flight. You may also wish to note your ground speed at this point, as it will come in handy later.

Next, you need to calculate your total time enroute. This topic is found in another FAQ, however I will touch the basics that apply only to
fuel calculation. Your total time enroute can be calculated by the formula: Time = Distance/Ground Speed. Ground speed is total air
speed corrected for wind. To calculate your estimated ground speed, consult a weather website such as the National Weather Service
from the USA that provides winds aloft charts for the majority of the world.

Once you have your estimated time enroute, fuel calculation is easy. For this example, I am going to use some rough numbers on a 747-
400 flight from RJAA (Tokyo Narita) to KMSP (Minneapolis/St. Paul, MN):

Total Estimated Enroute Time = 12:00
747-400 Fuel Rate of Burn = 24 000 lbs/hr
Total Trip Fuel Required = 24 000 * 12 = 288 000 lbs of fuel

Now once you have that number, add one hour for your reserve, or in this case, 24 000 lbs for a total of 312 000 lbs. Now, take that
number and add 10% to it for the 'unknowns' of the trip (such as slower ground speed than anticipated, etc) so we have 312 000 * .10 =
31 200 lbs + 312 000 lbs = 343 200 lbs total required fuel.

You may also add additional fuel for such items as taxi, holding, etc. if you wish, however, the formulas that I used above should be
sufficient for any trip.

On a final note, I am sure that the thought has crossed your mind: 'What about the extra fuel I burn on climbout?' Believe it or not, that if
you calculate the fuel you burn on climbout and then subtract the fuel that you burn on descent, you will come up with a number that is
VERY CLOSE to your cruise fuel rate of burn. In short, adding additional fuel for climb is not necessary.