Beginning with minimum flow rates, I calculate a Linear Water Losses Index (LWLI) for each district that I then compare to the data available from my local water authority—see below.
Where D represents the number of subscribers per network km and where the Linear Water Losses Index is m3/day/km.
There's also a more empirical method proposed by Lambert at http://www.leakssuite.com/concepts/uarl-and-ili/.
With this method, you set Unavoidable Annual Real Losses (UARL) as the minimum target. The UARL is a theoretical reference value that predicts what real losses would be for a given system if its infrastructure were in good condition and the best technology could be successfully applied. As a result, it is also a key variable in the calculation of the Infrastructure Leakage Index (ILI), which effectively attests to how well the distribution network is being managed and maintained.
The detailed info you need is in the link, but the basic formula is as follows:
UARL (liters/day) = (18 x Lm + 0.8 x Nc + 25 x Lc) x P
UARL (gallons/day) = (5.41 x Lm + 0.15 x Nc + 7.5 x Lc) x P
Lm = length of mains (km or mi.)
Nc = number of customer service connections
Lp = average length of customer service connection piping, property line to meter (m or ft.)
Lc = total length of customer service connection piping (km or mi.) Lc = Nc x Lp (km or mi.)
P = average pressure (m or psi)