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Calcimator

Rainwater Collection Calculator

Size rainwater harvesting tanks and estimate annual collection based on roof area, rainfall, and usage patterns.

Inputs

Results

Net Annual Collection

92,319 gal

≈ 7 backyard pools

Recommended Tank Size

4,200 gal

≈ 11 hot tubs

Days of Supply462 days
Pump Size10 GPM
How to Use This Calculator
  1. Enter Roof Area (sq ft), Annual Rainfall (in), and Daily Usage (gal).
  2. Set First Flush Diverter (gal).
  3. Review Net Annual Collection (gal) and Recommended Tank Size (gal).
  4. Use Days of Supply (days) and Pump Size (GPM) to inform your decision.
  5. Use the chart to visualize the results and explore different scenarios by adjusting inputs.

How the result changes with Roof Area (sq ft)

Roof Area (sq ft)Net Annual CollectionRecommended Tank Size
50,090951,906 gal4,200 gal
175,0653,334,404 gal4,200 gal
325,0356,193,402 gal4,200 gal
450,0108,575,901 gal4,200 gal

What each input means

Roof Area (sq ft)
Total roof catchment area in square feet
Annual Rainfall (in)
Average annual rainfall in inches for your location
Daily Usage (gal)
Expected daily non-potable water usage (irrigation, toilets, cooling)
First Flush Diverter (gal)
Gallons diverted per rain event to remove roof contaminants

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Roof Area (sq ft) = 5000, Annual Rainfall (in) = 36, Daily Usage (gal) = 200, First Flush Diverter (gal) = 50 = 4 input(s) provided
  2. Calculate Net Annual Collection
    92319 = 92319
  3. Calculate Recommended Tank Size
    Recommended Tank Size
    4200 = 4200
  4. Calculate Days of Supply
    Days of Supply
    462 = 462

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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