Rainwater Harvesting Calculator
Size rainwater collection and storage systems for non-potable use
Collection Area
Rainfall Data
Water Demand
System Requirements
Recommended Tank Size
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% Demand Met
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Annual Collection
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Monthly Average
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Daily Demand
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First Flush Diverter
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Note: Size storage for longest dry period demand plus 20% reserve.
System Components
- Gutters and downspouts
- First flush diverter
- Leaf screen/filter
- Storage tank with overflow
- Pump and pressure tank
- Backflow preventer
- Distribution piping (labeled)
Code Requirements
Non-Potable Systems:
- Purple pipe required for distribution
- Label all outlets "NON-POTABLE"
- Cross-connection protection
- Permit typically required
Typical Water Demands
| Use | Demand per Day | Annual (4 person household) |
|---|---|---|
| Toilet Flushing | 18-24 gal/person | 26,000-35,000 gal |
| Laundry | 10-15 gal/person | 14,000-22,000 gal |
| Irrigation (per 1000 sf) | 0.6 gal/sf/week | 2,500 gal/1000 sf |
| Car Washing | 50-100 gal/wash | 2,500-5,000 gal |
Rainwater Collection Formula
Gallons = Roof Area (sf) x Rainfall (in) x 0.623 x Collection Efficiency
0.623 = conversion factor (gallons per square foot per inch of rain)
Example: 2000 sf roof x 1" rain x 0.623 x 0.90 = 1,121 gallons per inch of rainfall