Rainwater Harvesting Calculator

Every 1 millimeter of rain falling on 1 square meter of catchment produces 1 liter of water before losses.

Rainwater potential (L) = catchment area (m²) × rainfall (mm)

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To account for collection losses: Estimated harvest (L) = rainwater potential × collection efficiency ÷ 100

 

For example, a 20 m² catchment receiving 400 mm of rain in a month has a theoretical collection of 8,000 liters. At 90% collection efficiency, the estimated harvest is 7,200 liters. Use the horizontal roof area that drains to your collection system, measured as if viewed from above. Some roofs drain toward several destinations, so include only the area feeding the tank you are assessing.

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What size rainwater tank do I need?

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Start with the amount of water you expect to use each day and the number of rainless days you want storage to cover. Daily household demand (L/day) = number of people × daily use per person. Required usable storage (L) = daily household demand × days of storage.  A household using 240 liters per day needs 2,400 liters of usable storage to cover 10 rainless days, assuming the tank starts full.

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The calculator rounds this requirement up to the next 500 liters as a starting capacity estimate. Check the tank’s actual usable volume: water below the outlet and other operating limits can reduce the amount available.

 

Frequently asked questions (FAQs)

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Can rainwater meet all my household water needs?

If you plan properly, yes! Compare estimated collection with household demand first. If demand exceeds collection over time, a larger tank alone may close the shortfall. You may need more catchment area, lower consumption or an additional water source. Even when average collection exceeds demand, rainfall timing and storage capacity determine whether water lasts through dry periods.

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What collection efficiency should I use?

Collection efficiency estimates the share of rainwater that reaches your tank after losses from splashing, gutters, leaks and first-flush diversion. The calculator starts at 90% as an illustrative assumption. Adjust it to reflect your roof and collection system. Tank overflow is not simulated.

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Can average monthly rainfall tell me the ideal tank size?

Average monthly rainfall helps estimate collection potential. It does not show whether rain arrives regularly or in a few heavy storms. More reliable tank sizing uses rainfall records across wet and dry periods, daily demand, usable storage capacity and overflow.

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Are US gallons and Imperial gallons the same?

No. One US gallon equals approximately 3.785 liters, while one Imperial gallon equals approximately 4.546 liters. The calculator labels both standards so you can compare tank capacities correctly.

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Can I use this for a homestead or permaculture design?

Yes. Use it to compare roof catchments, estimate household supply and explore storage needs during early design. The demand calculation uses household size and daily use per person. Irrigation, livestock and other site uses require additional demand estimates.

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Planning a water system?

Rainwater storage can work well when considered alongside roof drainage, water use, tank placement, treatment, maintenance and overflow routes. 

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Feel free to contact One Regeneration to get connected with a service provider who can discuss water management and ecological design for your project. 

 

This calculator supports education and preliminary planning. Its results do not guarantee continuous supply or establish water quality.