Learn · Updated 2026-09-29
Infrastructure Leakage Index (ILI) explained
The ILI compares your current real losses with the unavoidable minimum for your network. Learn the formula, the World Bank bands from A1 to D, and what your ILI tells you to do next.
The Infrastructure Leakage Index is the ratio of the real losses you have to the real losses you could not avoid even with excellent management. It is the internationally accepted way to compare leakage performance between systems of different size, pressure and layout.
ILI = CARL ÷ UARLWhat an ILI value means
An ILI of 1.0 means you are leaking at the technical minimum. An ILI of 4 means four times the unavoidable volume. The World Bank Institute banding system turns the number into an action:
| Band | Standard systems | Low & middle income | Interpretation |
|---|---|---|---|
| A1 | < 1.5 | < 2 | World class. Only marginal further reduction is possible. |
| A2 | 1.5 – 2 | 2 – 4 | Further reduction may be uneconomic unless water is scarce; careful analysis needed. |
| B | 2 – 4 | 4 – 8 | Marked improvement possible: pressure management, active leakage control, better maintenance. |
| C | 4 – 8 | 8 – 16 | Poor leakage record. Tolerable only if water is plentiful and cheap. |
| D | > 8 | > 16 | Highly inefficient. Leakage reduction is imperative and high priority. |
When ILI is not appropriate
The ILI assumes the UARL formula holds, which requires roughly 3,000 or more service connections, at least 20 connections per kilometre of mains and an average pressure of 25 m or more. Below those thresholds, litres per connection per day per metre of pressure is a more reliable comparison. Intermittent supply must also be corrected for — the calculator expresses everything 'when the system is pressurised' (w.s.p.).
Uncertainty on ILI
Because real losses are the residual of the water balance, the ILI inherits every uncertainty in the inputs, plus the uncertainty on mains length, connections, service-pipe length and pressure in the UARL. A well-run balance often produces an ILI with a ±20–40% band. Reporting that band is honest; hiding it is not.
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