Which area should I use to calculate the force on a submerged surface?

[rbmartel]

TL;DR

In order to find the resultant force applied to a submerged vertical wall, which area should I consider?

Let's say we have a tank filled with water only half way up. I want to calculate the force being applied by the liquid on one of the walls, that's F = P.A. For the area (A), should I consider the area of the entire wall (H.L), or only the area of the wall that's in contact with the liquid ((H/2).L).
I'm having a hard time trying to understand if I should or shouldn't include in the calculations the part of the wall that is not in contact with the water.

 

[anuttarasammyak]

You should take area in contact with liquid. If the upper part is in contact with air, you may have to consider air pressure on that part.

 

[kuruman]

You also need to consider that the total force is pressure times area only if the pressure is constant. Here the pressure is not constant because it increases with depth. You will have to do an integral.

 

[jbriggs444]

You also need to consider that the total force is pressure times area only if the pressure is constant. Here the pressure is not constant because it increases with depth. You will have to do an integral.

And, of course, an integral taken over the whole area will yield the same result as an integral taken over the area where the pressure is non-zero.