DIY Cost Tools
Rainwater Collection Calculator
HOME & OUTDOOR PLANNING
Estimate rainwater yield from horizontal catchment area, rainfall, and collection efficiency.
Use the units shown. All fields are required; absolute values cannot exceed one trillion. Storage capacity, overflow, first-flush diversion, evaporation, leaks, and changing rainfall are not modeled separately. Yield is not a water-quality assessment. Do not assume untreated collected water is potable.
How to use this calculator
Replace the example inputs with your own measured values, using the units beside each field. Select Calculate, then compare results under different practical assumptions. Keep extra precision in measurements until the final answer.
Formula and explanation
Theoretical US gallons = area(ft²) × rain(in)/12 × 7.48051948052; collected gallons = theoretical gallons × efficiency/100
Rainfall depth applies to the horizontal projected catchment area. Efficiency accounts for estimated runoff and collection losses.
Worked example
With Horizontal catchment area (square feet) = 1000, Rainfall depth (inches) = 1, Collection efficiency (%) = 80, the results are Theoretical rainfall volume (US gallons): 623.37662; Estimated collected volume (US gallons): 498.7013; Estimated collected volume (liters): 1,887.7898.
Important limitations
Storage capacity, overflow, first-flush diversion, evaporation, leaks, and changing rainfall are not modeled separately. Yield is not a water-quality assessment. Do not assume untreated collected water is potable.
These are planning estimates. Confirm actual measurements, product instructions, site conditions, and any applicable project requirements before purchase or installation.
Frequently asked questions
Should I use sloped roof area?
Use horizontal projected catchment area, not the larger sloped surface area.
Will the tank retain all of this water?
Only if there is enough available storage and the system captures it. Overflow reduces retained volume.
Can I mix measurement units?
No. Convert values to the units shown beside the fields first. The result labels identify the output units.
What happens when inputs are invalid?
Blank, negative where disallowed, nonfinite, or out-of-range values clear the old results and show an instruction.
Review notes
Reviewed October 1, 2026. Default examples were checked independently against the displayed equations and conversion factors. Page-script tests covered blank, negative, nonfinite, and excessive inputs. Calculations run locally in your browser.