| Month | Rainfall | Water harvested | Demand served | Ending storage | Shortage |
|---|
Why annual balance beats rule-of-thumb sizing
Many rainwater harvesting guides offer quick rules: multiply your roof area by annual rainfall and apply a runoff coefficient to estimate yearly yield. While that math is a good start, it hides the reality that rain arrives unevenly. A cistern that is perfectly adequate in a mild climate might run dry by late summer in a region with a pronounced dry season, even if total annual rainfall exceeds household demand. Conversely, oversizing a tank because of a single storm wastes money on storage that sits half empty. This planner simulates the month-by-month water balance so you can assess reliability, buffer capacity, and spillage. By pairing your own rainfall pattern with household demand, you gain a realistic sense of how many days of outage to expect and what interventions—such as conservation, backup wells, or larger tanks—would move the system toward the reliability you need.
Instead of lumping the year into a single number, the model tracks how each month’s rainfall fills the cistern, subtracts demand, and computes any shortages or overflow. The approach mirrors hydrologic mass-balance models used in utility planning, scaled down for household use. You can adjust the runoff coefficient to account for metal roofs, asphalt shingles, or first-flush diversion losses. Entering an emergency buffer requirement—expressed as days of demand—helps you determine whether the cistern dips below your comfort level and how often. The resulting table and CSV export provide a roadmap for maintenance scheduling, drought planning, and compliance with local rainwater harvesting regulations.
The basic water balance math
The planner converts all volumes into liters internally for consistency. Catchment yield is the product of roof area, rainfall depth, and the runoff coefficient. Let A represent the roof area, R the rainfall depth in meters, and C the runoff coefficient. The harvested water in cubic meters is A × R × C, which the calculator converts to liters. Monthly demand is daily demand multiplied by the number of days in the month. The MathML expression below summarizes the core step executed for each month:
In plain language, we start with the prior month’s storage, add the new water harvested, subtract demand, and cap the result at the tank’s maximum capacity. If demand exceeds what is stored, the cistern runs dry and the planner records the shortfall. By repeating this calculation for each month, the tool reveals how the tank drains and refills through wet and dry seasons, offering more nuance than annual yield estimates.
Worked example: sizing a cistern for an off-grid cottage
Consider a family of four living in a 1,900-square-foot cottage in the southeastern United States. The household averages 35 gallons of water use per person per day thanks to low-flow fixtures, yielding total demand of 140 gallons per day. Rainfall is plentiful in spring and summer but tapers during late fall. Using the default rainfall values and a 2,500-gallon cistern, the planner shows that the tank overflows during wet months yet drops below the three-day emergency buffer during October and November. Total annual demand is 51,100 gallons, while the roof yields around 58,000 gallons after accounting for the runoff coefficient. Despite the generous annual yield, the cistern experiences about 2,800 gallons of shortage scattered across late autumn and early winter because those months deliver less rainfall just as outdoor irrigation continues for garden beds.
The CSV output reveals the monthly pattern: storage peaks at nearly 2,400 gallons in July before irrigation and household use draw it down. By October, storage falls below 700 gallons, barely five days of demand, triggering the buffer warning. The family can respond in several ways. They could reduce outdoor watering in late summer, install a secondary storage tank to catch overflow in July, or add a backup well for drought months. The planner helps weigh those trade-offs by quantifying the shortage in units that translate directly to lifestyle: gallons missed and days of demand uncovered.
Interpreting the comparison between actual and ideal storage
Beyond the month-by-month simulation, the tool compares your existing tank to an “ideal” tank size that would capture all rainfall without overflow. It does this by running the same water balance with an effectively unlimited cistern to determine the maximum volume that ever accumulates. If your current tank is smaller than that peak, you are spilling water in wet months that could have supported dry-season reliability. The planner reports how much additional storage would be required to capture all rainfall and how much shortage would remain even with a perfect tank. If the shortage persists despite infinite storage, the issue is structural: the climate simply does not supply enough rainfall to meet the demand, signalling the need for conservation or supplemental sources.
Using the table and CSV export
The monthly table displays rainfall depth in your chosen units along with the water harvested, demand served, ending storage, and shortage volumes. This layout mirrors asset management spreadsheets used by architects and engineers. You can sort through the table to identify the months when shortages occur or when storage exceeds the tank’s emergency buffer requirement. The CSV export includes the same data plus calculated spill volumes and percent of demand served, letting you create charts or share the forecast with contractors. Comparing scenarios—such as adjusting the per-person use downward to simulate conservation—helps families and institutions decide whether to invest in behavior change or physical infrastructure.
Experimenting with conservation and buffering
Rainwater harvesting is inherently iterative. The planner encourages experimentation by allowing you to change daily use, household size, runoff assumptions, and starting fill levels. Drop the per-person use from 35 to 28 gallons to simulate aggressive conservation and observe how the shortage shrinks. Increase the starting fill level to 80% to model beginning a dry season with a full tank. Adjust the emergency buffer to five or seven days if you are far from backup supplies and note how frequently the tank dips below that threshold. Because the underlying math runs instantly in your browser, you can test dozens of combinations to arrive at a plan that balances cost and reliability.
Limitations and responsible interpretation
The planner assumes that rainfall is evenly distributed within each month and that all captured water is available immediately, which may not hold during intense storms that overwhelm first-flush systems. Evaporation, algae growth, and water quality treatment losses are not modeled. The runoff coefficient is treated as constant even though debris or leaf litter can reduce efficiency over time. Additionally, the tool uses a single year of rainfall averages; real weather varies year to year, so you should stress-test with a dry-year scenario. Despite these simplifications, the mass-balance framework captures the essential dynamics of storage and demand, providing a strong foundation for sizing cisterns, setting conservation targets, and scheduling maintenance in both residential and small commercial contexts.