With a gravity water feed system, a header tank is in the attic. Every 1 metre drop from the water tank typically equates to around 0.1 Bar in pressure.
There is a 500l cold water tank in the attic, the tank size is approximately 1.7m3. In the bathroom directly below the attic, the water pressure is low, around 0.15 bar; because those taps are less than 2metres below the tank.
I can gain an extra 0.1 bar by raising the existing tank 1m in the attic.
From what I understand about Newton gravity law the pressure would be measured from the surface of the water, so as the tank empties, the pressure falls.
Does the size and shape of the tank make a difference?
Pressure would increase when using a tall and narrow tank (presuming it is full). So instead of a 1,7m3 tank sitting at level 1m, or at level 2m, there would be a 2m tall cylindrical tank with 28cm radius, installed at level 1m.
The tank holds the same volume; the water pressure from the cylindrical tank would initially be higher, but decrease quicker as the water level fell. Would the pressure from a narrower tank be significantly different to a 1m high tank? Of course if the tank is almost empty, then having the base of the tank higher up would always be the best option.
I am wondering why water tank cisterns are not typically narrow and tall? They are easier to transport (up stairs and through the loft hatch) and it results in higher water pressure when full.