The Unit Line Flow Method divides the total demand in the system (or in a section of the system) into 2 parts: known demand (metered) and unknown demand (leakage and unmeasured user demand).

The following diagram shows a sample pipe. The known (metered) demands at nodes a and b are q_a and q_b respectively. The unknown demand is computed by considering if there are users on none, one, or both sides of the pipe. This is accounted for using the coefficient, K.

Where:

l_i = length of Pipe_i

K_i = coefficient indicating the capability of Pipe_i to consume water

If there are no users on either side of the pipe (the pipe is only used to transfer water to another part of the system), then K is 0. If there are users along only one side of the pipe (for example, pipes along a river), K is 0.5. If both sides of the pipe supply water to users, K is 1.

The equations below are used to determine the total demands at nodes a and b:

Where:

Q_a = the total demand at node a

Q_b = the total demand at node b

q_a = The known demand at node a

q_b = The known demand at node b

Q_{total unknown} = Total real demand minus total known demand(for the network or selection set)

n = number of pipes in network (or selection set)

m = the number of pipes connected to node a or b