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Glossary
Water standards and regulation

What is Infrastructure Leakage Index (ILI)?

Definition

The Infrastructure Leakage Index (ILI) is the ratio of current annual real losses to unavoidable annual real losses for a water distribution system, a dimensionless leakage score.

How the ILI is calculated

ILI = CARL / UARL. Current annual real losses (CARL) come out of the annual water balance after apparent losses are removed. Unavoidable annual real losses (UARL) are an estimate of the lowest leakage a well run network of that shape could reach, built from mains length, number of service connections, customer meter location and average operating pressure. An ILI of 1 means leakage is already at that floor.

Who reports it

Utilities report ILI inside annual water loss audits that follow the AWWA M36 method. In Texas, for example, the Texas Water Development Board requires retail public water systems with more than 3,300 connections, or with a financial obligation to the Board, to file a water loss audit every year, due by 1 May. Smaller systems file every five years.

What Aqueduct supplies to a real loss estimate

Aqueduct does not compute ILI; the utility or auditor does. What it supplies is the CARL side for a private or district network: minimum night flow and the unaccounted loss term per supply line, plus pressure sensor trends where they are fitted, kept as an hourly record instead of an annual estimate.

Questions

Infrastructure Leakage Index (ILI): common questions

The Infrastructure Leakage Index is a benchmark from the International Water Association's water loss work, adopted in the AWWA M36 audit method. It tells a utility how far its leakage sits above the technical minimum for its own network, which makes it fairer for comparison than raw loss volume. See non-revenue water.

Real losses are well above what that network could technically achieve. A high ILI usually points to slow repairs, high pressure or little active leak detection, so the utility's next steps are pressure management, faster repair and district metered areas to find where leakage runs.

A share of supply changes with demand and network size. ILI compares each network with its own unavoidable level, so a dense city and a rural system can be ranked fairly. Regulators and auditors therefore read ILI next to volume per connection rather than a single percentage.

Not meaningfully. The UARL inputs describe a distribution network of mains and service connections, so a single building is better served by its own water balance. Aqueduct gives that balance through the unaccounted loss term on each supply line, described under leak detection.

See Aqueduct on your own facility.

Bring a site, a utility bill or a question. We will show you where the loss is.

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