Start with a bounded question

Specify whether you want the potential from one storm, one month or one year. Each question needs rainfall covering that same period. An annual total is useful for screening a project, but it cannot tell you whether a tank is full during a particular dry week. Keep the rainfall source, location, period and measurement unit with the result.

Measure the contributing footprint

Use the horizontal area that drains to connected gutters, not the roof covering purchased from a roofer. If only one side of a pitched roof is connected, include only its horizontal contribution. Sketch separate roof sections and mark disconnected downpipes. This method lets another person check the estimate without needing to inspect hidden assumptions inside a calculator.

THE EXAMPLE AT A GLANCEYield changes with the assumed capture factor
Capture factor 0.6538,025 L/year
Capture factor 0.8549,725 L/year

An 11,700 L range is uncertainty in inflow, not guaranteed mains-water savings.

90 m² roof; 650 mm annual rainfall; illustrative overall factors.

Calculate a range, not a promise

For an illustrative 90 m² roof and 650 mm annual rainfall, the gross potential is 58,500 litres. Overall capture factors of 0.65 and 0.85 give 38,025 and 49,725 litres respectively. These factors are deliberately selected scenarios, not universal roof coefficients. The 11,700 litre difference shows why reporting a single precise-looking yield can obscure the important uncertainty.

Compare inflow with use

Suppose that household only needs 24,000 litres for permitted seasonal garden use. Even the higher inflow scenario cannot imply 49,725 litres of mains-water savings. Savings are capped by the water actually substituted, and may be further reduced by overflow or empty periods. Demand occurring before the rainy season cannot be supplied by rain arriving months afterward without adequate carryover storage.

Improve the estimate locally

Replace a distant annual average with nearby records and examine several years. Record the tank level before and after selected events, together with withdrawals. A consistently smaller rise may reflect connected area, wetting losses, diverted first water or restricted inlet flow. Investigate those causes before changing a capture factor to make the model look more optimistic.

Check the result before using it

  • Confirm millimetres rather than centimetres.
  • Exclude disconnected roof sections.
  • State whether the factor already includes filtration losses.
  • Show litres and cubic metres.
  • Compare available inflow with actual demand.
  • Examine a dry year.
  • Avoid equating collection potential with guaranteed annual savings.