3 Freeman's Formula
Q (L/min)= 1136.5(P/5+10)
Ministry of Urban Q (kilo liters/d)=100 √P for P>50000
4 Development Manual
Factors affecting per capita demand:
a. Size of the city: Per capita demand for big cities is generally large as compared to that
for smaller towns as big cities have sewered houses.
b. Presence of industries.
c. Climatic conditions.
d. Habits of people and their economic status.
e. Quality of water: If water is aesthetically & medically safe, the consumption will increase
as people will not resort to private wells, etc.
f. Pressure in the distribution system.
g. Efficiency of water works administration: Leaks in water mains and services; and
unauthorised use of water can be kept to a minimum by surveys.
h. Cost of water.
i. Policy of metering and charging method: Water tax is charged in two different ways: on
the basis of meter reading and on the basis of certain fixed monthly rate.
Fluctuations in Rate of Demand
Average Daily Per Capita Demand
= Quantity Required in 12 Months/ (365 x Population)
If this average demand is supplied at all the times, it will not be sufficient to meet the
Seasonal variation: The demand peaks during summer. Firebreak outs are generally
more in summer, increasing demand. So, there is seasonal variation .
Daily variation depends on the activity. People draw out more water on Sundays and
Festival days, thus increasing demand on these days.
Hourly variations are very important as they have a wide range. During active
household working hours i.e. from six to ten in the morning and four to eight in the
evening, the bulk of the daily requirement is taken. During other hours the requirement is
negligible. Moreover, if a fire breaks out, a huge quantity of water is required to be
supplied during short duration, necessitating the need for a maximum rate of hourly
So, an adequate quantity of water must be available to meet the peak demand. To meet all the
fluctuations, the supply pipes, service reservoirs and distribution pipes must be properly
proportioned. The water is supplied by pumping directly and the pumps and distribution system
must be designed to meet the peak demand. The effect of monthly variation influences the
design of storage reservoirs and the hourly variations influences the design of pumps and
service reservoirs. As the population decreases, the fluctuation rate increases.
Maximum daily demand = 1.8 x average daily demand