My overhead tank is at 12m height. From there a 50mm diameter pipe is running down and up to 50m horizontally and then raising from there to 6m to 3 bathrooms (no shower) and 3 wash basins. Is this height sufficient to supply water and how would I calculate the water pressure?
There are many online calculators for this, such as:
A meter of water is about 1.5 PSI so 12 meters is only 18 PSI and would be pretty disappointing for water pressure. You will have to run around in the shower to get wet.
When no water is flowing, the size of the pipe and the horizontal run are immaterial to this law of physics. It is the height of the water column that creates static pressure. When water is flowing, the pipe size and length may reduce the pressure, but in this case (assuming 50mm is an inside diameter) you shouldn't see any reduction in pressure.
you have to just look the net height difference between the water level in the overhead tank and exit point of pipe. as per your data net height differnce is 7 mtrs( involving 1 mtr water level in overhead tank). you can calculate pressure using formula P= density of water* acceleration due to gravity g* net height difference h (we have not considered the atmospheric pressure as it is balanced at water level and exit) when you open the tap the calculated pressure is converted into velocity v=square root of(2*g*h). it means water comes out with calculated velocity accompanied with atmospheric pressure. velocity will not depends upon the number of openings but the net height difference.