Formula and example
How it is calculated
Unexplained litres/hour = ((ending m³ − starting m³) × 1,000 − known water use in litres) / measurement hours. Annual m³ = litres/hour × hours/day × days/week / 7 × 365 / 1,000. Annual cost = annual m³ × (water price + wastewater share/100 × wastewater price).
Worked example
A meter change of 0.020 m³ over eight hours, with no known water use, represents 20 litres or 2.5 litres per hour. If that rate continues all day, every day, the possible loss is 60 litres per day and 21.9 m³ per year. At water and wastewater prices of 300 and 500 currency units/m³, with a 100% wastewater share, the annual cost is 17,520 currency units.
Things to check
- Meter movement does not prove the toilet is the source. Taps, pipes, irrigation and automatic appliances may also contribute. Exclude or account for known water use; unexplained flow is only a possible leak.
- Prices of 300 and 500 are illustrative inputs, not local tariffs. Enter the usage-based charges from your own bill and use the same currency throughout. Fixed standing charges and water heating are excluded.
- Daily results are averages across the week. The monthly estimate covers 30 days and the annual estimate 365 days, with the same leakage rate, schedule and prices throughout.
- Repair payback assumes the repair stops the whole measured loss. It excludes interest, future price changes and any damage caused by the leak.
Frequently asked questions
Does meter movement identify a leaking toilet?
No. It measures water passing through the meter. The source must be checked separately, and other water use during the measurement must be excluded or accounted for.
Why measure for longer?
A cistern that refills intermittently may not refill during a short test. A longer or repeated measurement can give a more representative average.