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Catchment yieldgal / LFree

Rainwater
Harvesting

Estimate how much rainwater a roof or catchment can collect over a year, from its area, the rainfall, and the runoff and collection efficiency.

Who it's for: architects, site engineers, and building owners sizing a cistern at design development or checking whether a roof catchment can meet irrigation or toilet-flushing demand.

Yield = area × rainfall × runoff × efficiency
US: gallons = area(ft²) × rainfall(in) × 0.623 × runoff coefficient × collection efficiency. Metric: litres = area(m²) × rainfall(mm) × runoff coefficient × collection efficiency. The displayed 0.85 runoff and 0.80 collection values are editable planning assumptions; confirm both for the selected roof and collection system.
Rainwater
Annual yield

How to use this calculator

1
Choose units
Pick Imperial (ft² + inches → gallons) or Metric (m² + mm → litres) to match your project drawings and rainfall data.
2
Enter catchment area
Enter the horizontal plan area of the roof or paved surface — use the footprint, not the sloped surface area.
3
Enter annual rainfall
Enter a representative annual precipitation depth for the site. In the US, use the NOAA 1991–2020 Climate Normal for a nearby station; elsewhere, use the current normal from the national meteorological authority.
4
Set the runoff coefficient
Enter a documented or conservatively selected factor for the actual catchment surface; the displayed 0.85 is only a planning assumption.
5
Set collection efficiency
Enter a project-specific factor for first-flush bypass, filter and gutter losses, and overflow; the displayed 0.80 is only a planning assumption.
6
Read the annual yield
The result is the theoretical harvestable volume for the year and a per-day average to compare against your irrigation or fixture demand.

The formula

The calculator converts rainfall over the catchment into a gross water volume, then applies the two user-entered loss factors.

US yield (gal/year) = A × P × 0.623 × C × E
Metric yield (L/year) = A × P × C × E
A = horizontal catchment area in ft² (US) or m² (metric)
P = representative annual precipitation in inches (US) or millimetres (metric)
C = user-entered runoff coefficient
E = user-entered collection efficiency
0.623 = 144 in³ per inch-foot² ÷ 231 in³ per US gallon, rounded to three decimals. In metric units, 1 m² covered by 1 mm of water is exactly 1 litre. The calculator does not prescribe C or E; its displayed values are editable planning assumptions.

Worked example

Example
For a 1,500 ft² catchment with 40 in/year of precipitation, C = 0.85, and E = 0.80, the implemented calculation is 1,500 × 40 × 0.623 × 0.85 × 0.80 = 25,418 gallons/year. Dividing by 365 gives a mathematical annual average of about 70 gallons/day. Tank sizing still depends on the timing of rainfall and demand.

When this estimate will be off

  • Assumes rainfall is distributed through the year — a short wet season with a long dry period requires a much larger cistern to bridge the gap.
  • Runoff coefficient is a single average; moss, debris, or an aging surface reduces actual collection below the stated value.
  • The default 0.80 collection efficiency is an editable planning assumption, not a verified value for a particular gutter, first-flush device, filter, or tank system.
  • Does not account for evaporation losses from open-top storage tanks or overflow during intense storm events above the cistern's capture rate.

Frequently asked questions

A common mistake: Assumes rainfall is distributed through the year — a short wet season with a long dry period requires a much larger cistern to bridge the gap.

Use a coefficient documented for the actual roof assembly when one is available. Surface texture, slope, wetting, debris, and leakage all affect runoff. The calculator's displayed values are editable screening assumptions, not universal material properties. A conservative project value should be confirmed against local guidance or measured system performance.

One inch of rain on one square foot equals 144 cubic inches of water. NIST Handbook 44 defines one US gallon as exactly 231 cubic inches. Dividing 144 by 231 gives 0.6233766 gallons, which the implemented formula rounds to 0.623 gallons per inch-foot².

Use a documented value from the selected system or a value justified by project-specific loss calculations. The default 0.80 is only a planning assumption. First-flush diversion, filter losses, gutter leakage, tank overflow, and maintenance can all change actual performance, so the entered efficiency should be tested during detailed design.

This calculator estimates collection volume; it does not determine water quality or approve an end use. Potable-treatment and permitting requirements vary by jurisdiction and system. Verify the applicable plumbing code and health-authority rules, and obtain a project-specific treatment design before connecting harvested water to any potable distribution system.

Sources

  • NIST Handbook 44, Appendix C — defines one US gallon as exactly 231 cubic inches, supporting the 0.623 gal per inch-foot² conversion
  • NIST Guide to the SI, Chapter 5 — defines one litre as 1 dm³ (10⁻³ m³), so 1 mm over 1 m² equals 1 litre
  • NOAA NCEI 1991–2020 U.S. Climate Normals — primary US source for representative annual precipitation normals; use a nearby station appropriate to the site

This free Rainwater Harvesting Calculator is built and maintained by DataDrivenAEC, using the relevant codes and standards. It does not substitute for professional judgment.