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💧 Rainwater Capture Calculator – Estimate Harvestable Water From Rainfall

Use this simple calculator to estimate how much rainwater you can collect from a roof or other catchment surface using rainfall, area, runoff coefficient, system efficiency, and first-flush loss.

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Net capture (gallons / year)
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Liters / year
0
55-gal barrels

This tool provides a practical estimate. Actual capture varies by rainfall pattern, surface condition, debris, overflow, first-flush settings, and storage system design.

Enter your rainfall and catchment details

Use annual rainfall totals where possible. For roofs, use the horizontal footprint area rather than the sloped surface area unless you are modeling a specific design.

  • Catchment area: square feet of roof or collection surface.
  • Rainfall: inches per year.
  • Runoff coefficient: how much rain actually runs off the surface.
  • Efficiency: accounts for filter, gutter, conveyance, and collection losses.

What’s included?

This quick estimator helps size potential water capture and storage value.
Gross capture
Net harvestable water
Tank fill cycles
Days of supply

Want this matched to your state or monthly rain pattern? A location dropdown and monthly storage model can be added next.

Inputs

Leave tank size and daily demand blank if you only want annual capture.
Use the roof footprint or collection surface area.
Choosing a surface auto-loads a runoff coefficient.
Typical range: 0.70 to 0.95 for roofs.
Accounts for gutter, filter, and transfer losses.
Use 0 if you want a simple no-loss estimate.

Formula factors (editable)

Default conversion uses the standard U.S. rainwater harvesting relationship: 1 inch of rain on 1 square foot yields about 0.623 gallons before losses.

Factor Value Unit
Rainfall conversion gallons per sq ft-inch
Liters per gallon liters / gallon
Gallons per barrel gallons / barrel

Results breakdown

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Gross capture (gal / year)
0
Net capture (gal / year)
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Average monthly capture (gal)
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Cubic meters / year
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Tank fills / year
0
Days of supply

Need an irrigation version next? This can be extended with crop water demand, growing season, and monthly rainfall storage modeling.

How the estimate works

Gross capture = catchment area × annual rainfall × conversion factor × runoff coefficient.

Net capture = gross capture × system efficiency × (1 − first-flush loss).

Tank fills and days of supply are optional planning outputs based on the tank size and daily water demand you enter.