How much fuel does a commercial plane require?

We all knew that airplane needs a huge amount of fuel depending on the type. But how would they calculate?

Actually, the thing is they fill the plane with fuel for various purposes. Firstly, there is taxi fuel to move the fuel across the taxiways during takeoff and landing. Secondly,  there is the Trip fuel for the flight which burnt down to transport a plane from departure to arrival area. Then, there is Contingency fuel for a case like crowded traffic and bad weather conditions to rumble for some time in the sky. So, the plane may be able to reach the nearby airport for a successful flight. Here comes Alternate fuel if successive airports nearby are overcrowded or affected by bad weather conditions.After that, there is Final Reserve fuel to reach the permanent destination from the temporary one. Including all of these, the plane takes off from the airport. The Ballast fuel is also there which is used to maintain Centre of Gravity and to avoid bending of the plane during landing within limits.