Rainwater Harvesting Calculator

Catchment System Parameters

Catchment Area

m²

100 m²

Typical house roof: 100-200 m²

Average Rainfall

mm/year

1,000 mm/year

US Average: Seattle (950mm), Phoenix (200mm), Miami (1,570mm)

Annual precipitation in your location

Roof Material / Runoff Coefficient

Metal Tile Asphalt Wood Gravel

coefficient

0.95

Portion of rainfall that becomes runoff (0-1)

Collection Efficiency

85%

High: 90%+ Medium: 75-90% Low: <75%

Accounts for evaporation, splash, gutter losses

Time Period

Monthly Yearly

Select calculation period

Harvesting Results

Harvestable Rainwater

80,750 L

Per year collection

Visual: 5,000L tank (fills ~16 times/year)

Recommended Tank Sizes

1,000L 2,500L 5,000L 10,000L 20,000L

5,000L tank recommended for balanced storage

Calculation Breakdown

Rainfall Volume	100,000 L
Runoff Coefficient	0.95
Collection Efficiency	85%
System Losses	19,250 L
Net Harvestable	80,750 L

This could supply...

Toilet flushing (6L/flush) 13,458 flushes

Garden watering (20L/m²/week) 77.6 m² garden

Laundry loads (70L/load) 1,154 loads

Monthly Distribution (Sample)

Jan

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Rainwater Harvesting Potential

How Rainwater Catchment Works

Rainwater harvesting is an ancient practice that's gaining modern relevance for water conservation, stormwater management, and sustainable living. Understanding the principles behind rainwater catchment helps you design more effective systems and maximize your water collection potential.

Catchment Calculation Formula

The fundamental rainwater harvesting formula is based on simple water balance principles:


                Harvested Water (L) = Catchment Area (m²) × Rainfall (mm) × Runoff Coefficient × Collection Efficiency

Where:

Catchment Area: The surface area from which rainwater is collected (typically roof area in square meters)
Rainfall: Depth of precipitation (1 mm = 1 liter per m² of surface area)
Runoff Coefficient: Portion of rainfall that becomes runoff (0-1, depends on surface material)
Collection Efficiency: System efficiency accounting for losses (evaporation, splash, first-flush diversion)

Worked Example

Let's calculate for a typical residential scenario:

Input Parameters:
- Catchment area: 100 m² (medium-sized house roof)
- Annual rainfall: 1,000 mm (moderate climate)
- Runoff coefficient: 0.95 (metal roof, high efficiency)
- Collection efficiency: 85% (well-designed system with losses)
Step 1 - Total Rainfall Volume: 100 m² × 1,000 mm = 100,000 L (1 mm rain on 1 m² = 1 liter)
Step 2 - Runoff Volume: 100,000 L × 0.95 = 95,000 L
Step 3 - Collection After Efficiency: 95,000 L × 0.85 = 80,750 L
Step 4 - Monthly Average: 80,750 L ÷ 12 = 6,729 L/month
Step 5 - Daily Average: 80,750 L ÷ 365 = 221 L/day

Key Factors Affecting Harvest Potential

Factor	Typical Range	Impact on Collection	Optimization Tips
Roof Material	Runoff coefficient: 0.7-0.95	Metal/tile roofs collect 20-30% more than gravel	Clean roof surface, consider roof coating
Gutter System	Efficiency: 80-95%	Proper slope and sizing prevents overflow	Size gutters for 100-year storm, regular cleaning
First-Flush Diverter	Loss: 5-20% of rainfall	Improves water quality but reduces quantity	Adjustable diverter based on roof contamination
Tank Storage	Size: 2,000-20,000L	Limits collection to tank capacity between uses	Size tank for dry period demand, consider串联 tanks
Climate Patterns	Seasonal distribution	Uneven rainfall requires larger storage	Supplement with other sources in dry seasons

Rainwater Quality Considerations

While this calculator focuses on quantity, water quality is equally important for rainwater harvesting systems:

Roof Contaminants: Bird droppings, leaves, dust, and atmospheric pollutants
Treatment Options: First-flush diversion, filtration (mesh, sand, carbon), disinfection (UV, chlorine)
End-Use Requirements: Toilet flushing/garden watering need less treatment than potable uses
Maintenance: Regular gutter cleaning, tank inspection, filter replacement

System Design Tips:

Size storage for 2-4 weeks of dry season demand
Include overflow provisions for heavy rain events
Consider串联 tanks to maximize space utilization
Install leaf screens and first-flush diverters for better water quality
Check local regulations - some areas restrict or regulate rainwater harvesting
For potable use, include proper filtration and disinfection systems

Frequently Asked Questions

How much rainwater can I realistically collect from my roof?

Realistic collection depends on several factors:

Roof Area: For every square meter of roof area, 1 mm of rain yields 1 liter of water (before losses)
Example Calculation: 100 m² roof × 1,000 mm annual rainfall = 100,000 L theoretical maximum
Typical Losses:
- Roof absorption/splash: 5-20% (depends on material)
- Gutter inefficiency: 5-10%
- First-flush diversion: 5-25% (for water quality)
- Evaporation: 2-5%
Realistic Yield: Typically 70-85% of theoretical maximum with a well-designed system
US Examples:
- Arizona (200mm rain): 100m² roof → ~14,000 L/year
- Georgia (1,200mm rain): 100m² roof → ~85,000 L/year
- Washington (950mm rain): 100m² roof → ~67,000 L/year

What size storage tank do I need?

Tank sizing involves balancing rainfall patterns with water demand:

Determine Daily Demand: Calculate how much rainwater you'll use daily (toilet flushing, irrigation, laundry, etc.)
Consider Dry Periods: Size tank to cover demand during longest expected dry spell (typically 2-4 weeks)
Rule of Thumb: Tank capacity = Daily demand × Days of dry period storage
Example:
- Daily demand: 200 L (toilet flushing + garden)
- Dry period: 21 days
- Minimum tank size: 200 L × 21 = 4,200 L
Practical Constraints:
- Space availability (underground vs. above-ground tanks)
- Budget (larger tanks cost more but provide more security)
- Overflow management (tank must have overflow outlet)
Common Residential Sizes:
- Small system: 1,000-2,500 L (supplemental use)
- Medium system: 5,000-10,000 L (primary non-potable use)
- Large system: 15,000-30,000 L (whole-house including potable with treatment)

Is rainwater safe to drink?

Rainwater can be safe to drink with proper treatment, but untreated rainwater carries risks:

Contamination Sources:
- Roof materials (asphalt shingles may leach chemicals)
- Bird/rodent droppings
- Atmospheric pollution (industrial areas)
- Dust, pollen, leaves
Pathogens of Concern: Bacteria (E. coli, Salmonella), viruses, protozoa
Treatment Requirements for Potable Use:
1. First-flush diverter: Diverts initial contaminated runoff
2. Filtration: Mesh/sediment filters → carbon filters
3. Disinfection: UV light, chlorination, or ozone treatment
4. Regular Testing: Especially for bacterial contamination
Safer Non-Potable Uses (minimal treatment):
- Toilet flushing
- Garden/landscape irrigation
- Car washing
- Laundry (with appropriate filtration)
Regulatory Status: Many local health departments restrict or regulate rainwater for drinking; always check local codes

What maintenance does a rainwater harvesting system require?

Regular maintenance ensures system efficiency and water quality:

Monthly:
- Clean gutter screens and leaf guards
- Check first-flush diverter (clean, adjust if needed)
- Inspect tank overflow and mosquito screens
Seasonally (every 3-6 months):
- Clean gutters and downpipes thoroughly
- Check and clean pre-tank filters
- Inspect tank interior (via access port) for sediment accumulation
- Test water quality if used for sensitive applications
Annually:
- Complete system inspection
- Clean tank if significant sediment has accumulated
- Service pumps and pressure systems
- Replace filter cartridges as needed
- Check all connections and seals
After Heavy Storms: Check for debris accumulation, ensure proper drainage
Winter Considerations (cold climates):
- Drain above-ground pipes before freezing temperatures
- Consider tank insulation or buried installation
- Use freeze-proof fixtures

Are there financial benefits to rainwater harvesting?

Financial benefits vary by location, system scale, and water costs:

Direct Water Bill Savings:
- Average US water cost: $1.50-3.00 per 1,000 liters
- Example: 80,000 L annual collection = $120-240 savings/year
- Higher in areas with expensive water (California, Southwest)
Indirect Benefits:
- Reduced stormwater fees (some municipalities offer credits)
- Lower irrigation costs for landscaping
- Increased property value (sustainability feature)
- Drought resilience (avoid water restrictions)
Cost Considerations:
- Simple barrel system: $100-500 (DIY)
- Intermediate system (5,000L tank, gutters, pump): $2,000-5,000
- Whole-house system with treatment: $8,000-20,000+
- Payback period: Typically 5-15 years depending on system cost and water prices
Incentives & Rebates:
- Some states (Texas, Arizona, California) offer tax credits or rebates
- Check with local water utilities for conservation programs
- Federal tax deductions may apply for certain water conservation improvements
Non-Financial Benefits: Water independence, environmental stewardship, educational value, stormwater management

Quick Harvesting Tips

Clean gutters regularly
Use first-flush diverter
Size tank for dry periods
Check local regulations
Consider water end-use
Maintain system annually

Roof Runoff Coefficients

Metal roof: 0.90-0.95
Tile roof: 0.85-0.90
Asphalt shingles: 0.80-0.85
Wood shingles: 0.75-0.80
Gravel roof: 0.70-0.80
Green roof: 0.30-0.50