Water purification, distribution and sewage disposal for Peace Corps volunteers 
Section 4: Characteristics of an adequate system 

HEAD
In planning a distribution system it is desirable to draw water et a tap with a good pressure not too high nor too low. Required head is the height (or depth) of water which is required to produce a given pressure. Pressures are expressed in pounds per square inch (psi) or In height units (ft.,
Relation of Pressure and head: from the definition of head and some hydraulic factors, it can be shown that 2.31 ft. of water exerts 1 psi at its base or 7 psi exerted at base of water solumn will raise it 2.31 ft.
Example:
Calculate the pressure exerted by a column of water 100 ft et its base.
2.31 ft. exert 1 psi
100 ft. exert 1/2.31 x 100 psi = 43.3 psi
HEADLOSS
This is the reduction of pressure in a pipe which may be due to friction in pipe and pipe fittings, or valves, and can be expressed as a change in head. Allowable headless is the difference, in feet of water, between the tank election, and the elevation of service connection, plus the required head at the service connection. i.e. allowable head loss Elev. tank (elev. service connection + required head)
Example:
Given elevation of tank = 750 ft.
elevation service cone. = 665 ft.
required head (20 psi) = 46.2 ft.
allowable headloss = 750  (665 + 46.2) = 38.8 ft. of head.
Note:
The required head recommended by U.N. survey must be et least 10 psi (13.1 ft) for small water supply systems. For multistoried houses, minimum should be 70 psi (91.7 ft). 20 psi (46.2 ft) is a reasonable figure to work with.
Actual headloss is the headloss which actually occurs in pipe and joints. It must not exceed the allowable headloss. The pipes and joints selected must have a total headloss less than the allowable headloss.