About the Cistern Sizing
The Rainwater Cistern Sizing Calculator helps estimate two separate but related numbers: how large a storage tank you'd need for a target number of backup days, and how much rainwater your roof could realistically collect in an average month. It's meant for anyone planning a rainwater harvesting system for irrigation, non-potable household use, or drought backup.
How It Works
You enter your roof's catchment area, your average monthly rainfall, how many days of storage you want as backup, and your typical daily water draw from the cistern. The recommended cistern capacity comes directly from multiplying daily use by the number of storage days, while a separate figure estimates monthly collection potential from your roof area and rainfall using a standard runoff conversion and a collection efficiency factor.
Formula & Methodology
The 0.623 figure converts one inch of rain falling on one square foot of roof into gallons; multiplying by 85% accounts for typical losses from first-flush diversion, gutter inefficiency, and overflow. At the default 1,500 square foot roof and 3.5 inches of monthly rainfall, that's 1,500 x 3.5 x 0.623 x 0.85, or about 2,780 gallons of monthly collection potential. Separately, the recommended tank size at 20 gallons of daily use over 14 days of desired storage comes to 280 gallons, a figure derived purely from usage and storage duration, independent of rainfall.
Examples
Standard Residential Setup
With a 1,500 square foot roof, 3.5 inches of average monthly rainfall, 14 days of desired storage, and 20 gallons of daily use, the calculator recommends a 280-gallon cistern and estimates about 2,780 gallons of monthly collection potential.
Larger Roof, Longer Backup Window
A 2,000 square foot roof with 4 inches of average monthly rainfall, 21 days of desired storage, and 25 gallons of daily use yields a recommended 525-gallon cistern and roughly 4,236 gallons of estimated monthly collection.
Advantages
- Separates tank sizing based on your usage needs from collection potential based on your roof and climate, two distinct questions that are easy to conflate.
- Uses a standard, widely cited runoff conversion factor so the collection estimate is grounded in an established rule of thumb rather than a rough guess.
- Lets you test different storage-duration targets quickly to see how tank size scales before shopping for a physical cistern.
Common Mistakes
- Assuming the recommended cistern size accounts for rainfall automatically, when it's calculated purely from daily use and storage days and should be checked against the separate collection figure.
- Entering total roof square footage that includes areas not actually draining into the collection system, inflating the monthly collection estimate.
- Using a single average monthly rainfall figure without checking seasonal variation, which can make a system appear adequately sized on paper but leave it short during a dry stretch.
Edge Cases to Watch For
- Recommended cistern capacity is driven entirely by your daily use and desired storage days; it does not check whether your roof and local rainfall can actually refill that capacity between storage cycles, so pair it with the monthly collection figure to confirm feasibility.
- The 85% collection efficiency factor is a general assumption; systems without a first-flush diverter or with poorly maintained gutters will collect less than this figure implies.
- Monthly rainfall varies significantly across seasons, so a single average-month input can understate collection potential in wet months and overstate it during dry ones, and a full design should check month-by-month rainfall data.
Common Use Cases
- Homeowners in rural or drought-prone areas sizing a cistern for irrigation or emergency backup water.
- Off-grid or rainwater-dependent households planning a primary or supplemental water storage system.
- Landscape and rainwater harvesting designers producing an initial sizing estimate before a detailed system design.