Rainwater Harvesting Tank Size Calculator (Simplified)
Estimate potential rainwater harvest from your roof catchment area and calculate recommended storage tank size based on daily water demand and desired autonomy days. This is an educational planning tool only, not a substitute for professional plumbing design or water quality assessment.
Roof Catchment Area
Horizontal projection of roof area that drains to collection
Annual Rainfall
Total yearly rainfall for your location
Efficiency Factors
Metal: 0.9, Tiles: 0.8, Shingles: 0.75-0.85
Gutter/filter system efficiency (0.8-0.95 typical)
Water Demand & Storage
Typical: 50-100 L/person/day for non-potable use
Days of supply during dry periods (7-30 typical)
Site Context (Optional)
Rainwater Harvesting Calculator
Enter your roof catchment area, annual rainfall, and water demand to estimate potential rainwater harvest and recommended tank size.
Understanding Rainwater Harvesting
Rainwater harvesting is the collection and storage of rainwater from surfaces such as roofs for later use. This calculator helps estimate potential harvest volumes and storage tank sizes for planning purposes.
Key Concepts
Catchment Area
The horizontal projected area of the roof that drains to your collection system. Larger areas capture more water but require larger gutters and storage.
Runoff Coefficient
The fraction of rainfall that runs off the roof surface. Metal roofs (~0.9) shed more water than porous surfaces like concrete tiles (~0.75-0.8).
Collection Efficiency
Accounts for losses in gutters, downspouts, and filters. Well-designed systems achieve 85-95%. First-flush diverters reduce efficiency but improve water quality.
Storage Autonomy
Days of water supply during dry periods. This determines tank size based on daily demand. Longer dry seasons require more storage capacity.
Harvest Formula
Annual Harvest (L) =
Roof Area (m²) × Annual Rainfall (mm) × Runoff Coefficient × Collection Efficiency
Note: 1 mm of rain on 1 m² = 1 liter
Tank Sizing
Tank Size (L) =
Daily Demand (L) × Autonomy Days × (1 + Safety Margin)
Common Uses for Harvested Rainwater
- Garden and landscape irrigation
- Toilet flushing and laundry (with proper treatment)
- Car washing and outdoor cleaning
- Livestock watering
- Fire protection reserve
- Cooling tower makeup water
Important Considerations
- Water Quality: Rainwater may contain contaminants from roof materials, bird droppings, and atmospheric pollution. Treatment may be required for intended use.
- First Flush: The first portion of rainfall washes accumulated debris from the roof and should be diverted away from storage.
- Seasonal Variation: This calculator uses annual averages. Actual availability varies with seasonal rainfall patterns.
- Local Regulations: Check local building codes and health regulations for rainwater harvesting requirements.
- Potable Use: Using rainwater for drinking requires proper filtration, disinfection, and typically regulatory approval.
Frequently Asked Questions
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