💧 Rainwater Harvesting Calculator
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1 Catchment
2 Storage
3 Irrigation
4 Costs
5 Environment
0 gal
Annual Harvest
0 gal
Tank Size
0%
% Demand Met
$0
Annual Savings
— yrs
Payback
0 gal
Deficit/Surplus
1
Catchment Area & Rainfall
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Yellow = your primary inputs
Green = site/data inputs
Blue = calculated results
Annual rainfall:
>45 in (High)
25–45 in (Moderate)
10–25 in (Low)
<10 in (Very Low)
☝ Click a station dot to auto-fill local rainfall data.
Selected Location
— No location selected —
Apply Rainfall Data to Step 1
💧 Rainfall data applied
Auto-fills:
annual rainfall used to compute harvestable gallons and first flush estimates.
Roof / Catchment Specifications
Annual Rainfall (in/yr)
Roof / Catchment Area (sq ft)
Roof Material
Metal Roof (0.95)
Asphalt Shingle (0.85)
Green / Living Roof (0.40)
First Flush Diverter Size (gal)
Calculated Catchment Results
Runoff Coefficient
Annual Harvestable (gal, gross)
Annual Harvestable After First Flush (gal)
2
Storage Tank Sizing
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Yellow = your primary inputs
Green = site/data inputs
Blue = calculated results
Tank size is based on your daily demand and desired days of storage. Use multiple tanks for larger volumes.
Daily Water Demand (gal/day)
Days of Storage Desired
Tank Fill Rate (% of year rain falls)
Tank Size Needed (gallons)
Tank Size Needed (cubic feet)
Number of 500-gal Tanks
Number of 1,500-gal Tanks
3
Greenhouse Irrigation Demand
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Yellow = your primary inputs
Green = site/data inputs
Blue = calculated results
Crop water demand varies significantly. Leafy greens need far less water than fruiting crops. Irrigation efficiency accounts for system losses.
Greenhouse Area (sq ft)
Crop Type
Leafy Greens (0.15 gal/sqft/day)
Tomatoes / Peppers (0.25 gal/sqft/day)
Root Vegetables (0.20 gal/sqft/day)
Mixed Crops (0.20 gal/sqft/day)
Growing Season (months/yr)
Irrigation Efficiency (%)
Daily Irrigation Demand (gal/day)
Annual Irrigation Demand (gal)
% of Demand Met by Harvested Water
Deficit (−) or Surplus (+) gal/yr
4
Cost Analysis
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Green = your inputs
Blue = calculated results
System costs include tanks, gutters, and filtration. Annual savings are based on avoided municipal water costs using your net harvestable volume.
Equipment Costs
Cost per 500-gal Poly Tank ($)
Cost per 1,500-gal Poly Tank ($)
Gutters & Downspouts ($)
Filtration System ($)
Municipal Water Rate ($/1,000 gal)
Financial Results
Tank Equipment Cost
Total System Cost
Annual Water Cost Savings
Payback Period (years)
5
Environmental Impact
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Water treatment & distribution uses ~1.5 kWh per 1,000 gallons (EPA). Harvested rainwater bypasses that process. CO₂ factor: 0.386 kg/kWh (U.S. grid avg).
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0
Gallons of rainwater harvested / year
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0
Gallons of stormwater diverted / year
⚡
0
kWh of treatment energy saved / year
🏭
0
kg CO₂ avoided / year
🌳
0
Equivalent trees planted / year