Rain Barrel Calculator

What is Rainwater Harvesting?

Rainwater harvesting is the collection and storage of rain from roofs and other surfaces for later use. This ancient practice has gained renewed interest as homeowners seek to reduce water bills, conserve municipal water supplies, protect gardens during droughts, and reduce stormwater runoff that contributes to flooding and pollution.

Rain barrels are the simplest form of rainwater harvesting, connecting to downspouts to capture water from roof runoff. This calculator helps you determine your collection potential, size your system appropriately, and estimate the environmental and financial benefits.

Calculating Collection Potential

Collection Efficiency

Not all rain that falls on your roof reaches the barrel. Typical efficiency is 75-85% due to:

• Evaporation before reaching gutters
• Gutter splash and overflow
• First-flush diverters (if installed)
• Downspout filter losses

Sizing Your System

How Many Barrels Do You Need?

The number of barrels depends on your collection potential and intended use. A common mistake is underestimating how quickly barrels fill during rain events:

Roof Area	Gallons per 1/2" Rain	Recommended Barrels
500 sq ft	156 gal	2-3
1,000 sq ft	312 gal	3-4
1,500 sq ft	468 gal	4-6
2,000 sq ft	623 gal	6-8

Linking Multiple Barrels

Multiple barrels can be connected in series using overflow connectors, allowing excess water to flow from one barrel to the next. This maximizes storage without requiring larger individual containers.

Regional Rainfall Considerations

Seasonal Timing

Collection potential varies significantly by season. In many regions, spring and fall provide the most rain while summer (when you need water most) may be drier. Consider:

• Storing water from wet seasons for dry periods
• Using larger storage tanks for seasonal bridging
• Multiple smaller barrels at different downspouts

Water Usage by Application

Garden Irrigation

A typical vegetable garden requires about 1 inch of water per week during the growing season, equivalent to approximately 0.6 gallons per square foot. A 100 sq ft garden needs about 60 gallons weekly.

Lawn Watering

Lawns typically need 1-1.5 inches of water per week, making them intensive water users. A 1,000 sq ft lawn requires 600-900 gallons weekly - more than most rain barrel systems can provide alone.

Container Plants

Potted plants generally need less total water but more frequent watering. Rainwater is excellent for container plants because it lacks the chlorine and minerals found in municipal water.

Environmental Benefits

Stormwater Management

Rain barrels capture water that would otherwise become stormwater runoff. This runoff picks up pollutants (oil, fertilizers, pesticides) and carries them to waterways. Every gallon captured is a gallon that doesn't contribute to:

• Stream erosion and sediment pollution
• Combined sewer overflows
• Flooding in downstream areas
• Pollution of lakes, rivers, and oceans

Municipal Water Conservation

Using rainwater for outdoor purposes reduces demand on municipal water systems, which is especially valuable during drought conditions and peak summer usage. This saves the energy required for water treatment and distribution.

Plant Health Benefits

Rainwater is actually better for plants than tap water because:

• It's naturally soft (no mineral buildup in soil)
• It contains no chlorine or chloramine
• It has a slightly acidic pH that most plants prefer
• It contains trace nitrogen from the atmosphere

System Components

Essential Components

• Rain barrel or tank: 50-65 gallons typical
• Downspout diverter: Connects barrel to gutter system
• Overflow fitting: Directs excess water away
• Spigot: For hose connection or bucket filling
• Screen/filter: Keeps debris and mosquitoes out

Optional Enhancements

• First-flush diverter: Discards initial dirty runoff
• Elevated stand: Increases water pressure
• Pump: For longer hose runs or sprinklers
• Level indicator: Shows water level without opening

Installation Tips

Location Selection

1. Position under a downspout with good roof drainage
2. Place on stable, level ground
3. Ensure overflow drains away from foundation
4. Consider proximity to garden/use area
5. Provide shade if possible to reduce algae growth

Maintenance Requirements

• Clean screens and filters regularly
• Check for mosquito larvae monthly
• Drain and store barrels before freezing weather
• Inspect connections and seals annually
• Clean interior every few years

Frequently Asked Questions

Is rainwater harvesting legal?

Laws vary by state and municipality. Most states allow residential rainwater collection, though some Western states have restrictions based on water rights. Check local regulations before installing.

Can I drink collected rainwater?

Rainwater from rooftop collection systems should not be consumed without proper treatment. Roofs, gutters, and barrels can introduce contaminants. Rain barrels are intended for outdoor, non-potable use.

How do I prevent mosquitoes?

Keep screens in good condition, ensure overflow drains properly, and use the water regularly. If standing water occurs, biological mosquito dunks (Bti) are safe and effective.

How much money will I save?

Savings depend on water rates and usage. At $5 per 1,000 gallons, using 2,000 gallons annually saves about $10. The benefits are primarily environmental, though savings increase with higher water rates.