Rainwater Harvesting Calculator

How the Rainwater Harvesting Calculator Works

Rainfall depth multiplied by horizontal catchment area gives the theoretical volume of rain falling on the roof. For metric units, 1 mm of rainfall on 1 m² equals 1 liter. The calculator then applies roof runoff efficiency and, in Advanced Mode, additional collection losses and an optional per-event first-flush deduction.

Collection: Roof area × rainfall × runoff coefficient × (1 − additional loss fraction). Advanced first flush: subtract event count × first-flush volume, never below zero, for each month. Storage: the daily model distributes monthly collectible water across evenly spaced assumed rain events, caps storage at tank capacity, records overflow, and deducts daily demand, tracking any unmet demand.

Did You Know?

Using a sloped roof’s full sheet area instead of its horizontal projected area can overstate rainwater collection. Rainfall is measured as depth on a horizontal surface. A large annual rain total can also conceal several low-rainfall months.

How Much Rainwater Can I Collect From My Roof?

For a quick roof rainwater collection calculation, multiply the roof’s horizontal catchment area by rainfall depth and the collection efficiency. A 1,000 sq ft roof receiving 1 inch of rain has approximately 623 US gallons of theoretical runoff. At 85% collection efficiency, the usable collection estimate is about 530 US gallons, before any separate first-flush or additional system losses.

Use the rainwater collection calculator above to enter your roof area and rainfall in either square feet and inches or square metres and millimetres. It converts results into gallons or litres, making it useful for off-grid rainwater harvesting and initial planning of a household rainwater tank.

How Big Should My Rainwater Storage Tank Be?

A roof rainwater harvesting estimate tells you how much water could be collected, but not how much can be stored or supplied during dry weather. For rainwater tank sizing, enter daily household water demand and tank capacity, then use Advanced Mode with local monthly rainfall to compare overflow and unmet demand. Because the model assumes evenly spaced rain events within each month, use its storage results as planning estimates rather than a guarantee of water supply.

Worked Example

A home with a 100 m² horizontal roof catchment receives 1,000 mm of annual rainfall. At 80% roof runoff efficiency, estimated annual collection is 80,000 L before any additional first-flush or system losses. At 150 L/day, demand is 54,750 L/year, so annual theoretical coverage is 100%, with 25,250 L more collected than annual demand. That surplus does not prove that a 5,000 L tank will provide uninterrupted supply in dry months.

How to Use Your Results

• Confirm roof footprint and obtain location-specific monthly rainfall records.
• Use conservative runoff values where roofs are rough, contaminated, heavily shaded by trees, or have inefficient guttering.
• Size rainwater storage against the longest credible dry spell, not annual yield alone.
• Include pumps, screens, overflows, mosquito-proofing and safe treatment appropriate to the intended water use.

Expert Tips and Planning Advice

Collect rainwater from suitable roofing materials and prevent contamination from debris, animals, roof coatings, and nearby pollution. For drinking water, use proper assessment, filtration and disinfection, and follow local public-health guidance. This calculator estimates volume only; it does not certify potability or determine plumbing code compliance.

Annual Average vs. Seasonal Planning

Simple Mode is best for screening collection potential. Advanced Mode uses a daily-step assumed-storm model for comparing wet and dry months, testing proposed tank sizes, identifying overflow and unmet demand, and checking how much backup water may be needed. Neither mode models individual storms, evaporation from open storage, uncertain rainfall patterns, or climate extremes.

Rainwater Harvesting Frequently Asked Questions