Rainwater Collection Calculator

Design your complete rainwater harvesting system. Calculate collection potential, size your tank, and estimate cost savings with our comprehensive calculator.

System Design Calculator

Units:

Collection Surfaces

System Configuration

Intended Water Uses

Complete Guide to Rainwater Harvesting

System Components

  • Collection Surface: Usually a roof, but can include driveways and patios
  • Gutters & Downspouts: Channel water from roof to storage
  • First Flush Diverter: Diverts initial dirty water away from tank
  • Filters: Remove debris before water enters tank
  • Storage Tank: Stores collected rainwater
  • Overflow System: Manages excess water during heavy rain
  • Pump & Distribution: Delivers water to end uses

Runoff Coefficients Explained

Not all rain that falls on a surface is collected. The runoff coefficient represents the percentage of rainfall that becomes runoff:

  • Metal roofs (95%): Smooth surface, minimal absorption
  • Asphalt shingles (85%): Some absorption and evaporation
  • Concrete (85%): Mostly impervious
  • Grass (35%): High absorption and infiltration

Tank Sizing Considerations

  • Rainfall patterns: Consider seasonal variations
  • Water demand: Match storage to usage needs
  • Available space: Physical constraints on property
  • Budget: Larger tanks cost more but provide better security
  • Local regulations: Some areas have size restrictions

First Flush Calculations

The first flush diverter removes the initial flow of water that may contain contaminants from the roof. Industry standard is 0.0125 gallons per square foot of roof area (0.5 liters per square meter).

Water Quality & Uses

Non-potable Uses (No Treatment Required):

  • Garden and lawn irrigation
  • Toilet flushing
  • Car washing
  • Pressure washing
  • Pool top-off

Potable Uses (Treatment Required):

  • Drinking water (requires filtration and UV/chlorine treatment)
  • Cooking and food preparation
  • Bathing and showering

Maintenance Requirements

  • Gutters: Clean 2-4 times per year
  • First flush: Empty after each rain event
  • Filters: Clean or replace per manufacturer specs
  • Tank: Inspect annually, clean every 3-5 years
  • Mosquito prevention: Ensure all openings are screened

Local Regulations

Check with your local authorities before installing a rainwater harvesting system. Regulations vary by location and may include:

  • Permit requirements
  • Tank size restrictions
  • Setback requirements
  • Cross-connection prevention
  • Water rights considerations

Environmental Benefits

  • Reduces stormwater runoff and erosion
  • Decreases demand on municipal water supply
  • Lowers energy use for water treatment and distribution
  • Provides chlorine-free water for gardens
  • Helps recharge groundwater when used for irrigation

Garden questions, answered

Common questions before you buy materials

Use these quick answers alongside the calculator so you can plan the right amount, avoid waste, and keep your garden project moving.

How much rainwater can a roof collect?
Collection depends on roof area, rainfall, and system losses. Even a small roof can collect many gallons during a steady rain, but actual usable water is lower after overflow and first-flush loss.
Can I use rain barrel water on edible plants?
Use rain barrel water at soil level and avoid wetting edible leaves or fruit. Local guidance varies, so use extra caution with roof runoff and food crops.
How large should a rain barrel be?
Choose a size based on roof area, rainfall, and how quickly you use water. A larger tank helps through dry spells, but it still needs a safe overflow route.