Learn · Updated 2026-09-29
AWWA M36 water audit: how to calculate ILI and validated water loss in the US
The AWWA M36 water audit uses the IWA water balance with US terminology, gallons and psi. How the audit is structured, how data validity grading works, what state regulators expect, and how to run one online.
The AWWA Manual M36, Water Audits and Loss Control Programs, is the North American implementation of the IWA water balance. Several states — including California, Georgia, Texas and Tennessee — require utilities to submit an M36-style audit annually, and many require it to be validated by a certified validator before submission.
The structure
Water supplied is volume from own sources plus water imported, less water exported, each with a master-meter error adjustment. Authorised consumption is billed metered, billed unmetered, unbilled metered and unbilled unmetered. Apparent losses are unauthorised consumption (default 0.25% of water supplied), customer metering inaccuracies and systematic data-handling errors (default 0.25% of billed metered). Real losses are the residual. Non-revenue water is water supplied less billed authorised consumption — exactly the IWA definition.
Indicators M36 reports
| Indicator | Units | IWA equivalent |
|---|---|---|
| Real losses per service connection per day | gal/conn/day | L/conn/day |
| Real losses per mile of mains per day | gal/mile/day | m³/km/day |
| Real losses per connection per day per psi | gal/conn/day/psi | L/conn/day/m |
| Unavoidable annual real losses | gal/day | UARL |
| Infrastructure Leakage Index | ratio | ILI |
| Non-revenue water as % by volume and by cost | % | NRW % |
Data validity grading
Each input is graded 1 to 5 for the quality of the data behind it — from 'gross estimate' to 'metered with calibrated instruments and audited records'. The grades roll up to a data validity score out of 100. Scores below 50 mean the audit is a rough estimate; above 70, the indicators are considered reliable enough to set targets. WaterBalanceCalc's explicit ± bands play the same role and propagate directly into the uncertainty on real losses and ILI.
Running an M36 audit online
Switch to US customary units: volumes in million gallons, mains in miles, pressure in psi, tariff per thousand gallons. Enter exported water in the bulk-export rows. The results page reports gallons per connection per day and the ILI alongside the IWA indicators, and the Excel export lines up with the M36 reporting worksheet.
Put it into practice
Run your own water balance and see these indicators for your system — free, in about ten minutes.
Start a water balance