Rainwater Cistern Reliability Planner

How this rainwater cistern reliability planner works

This tool models a household rainwater harvesting system over a typical year so you can understand how often your cistern may run low, how much storage you need, and how sensitive the system is to your water use. Instead of relying on a single annual-average calculation, the planner steps through each month, tracking how rainfall fills the tank and how daily demand draws it down. The result is a simple but informative reliability check for off-grid homes, drought-prone properties, cabins, and backup water systems.

You provide four main types of inputs:

• Unit system: Imperial (square feet, inches, gallons) or metric (square metres, millimetres, litres). Internally the calculator converts everything to litres for consistent maths.
• Catchment and tank: Roof area, runoff coefficient, and cistern storage capacity, plus the starting fill level as a percentage of capacity.
• Demand: Household size and daily use per person, which together define your average daily draw from the tank.
• Climate: Average monthly rainfall depths for January through December at your site.

For each month, the calculator estimates how much rain falls on the roof, how much of that realistically becomes stored water after losses, and how much water the household uses. Storage is updated month by month, shortages are counted whenever the tank runs dry, and any water that arrives when the tank is already full is tracked as overflow or spillage.

Core water balance formulas

The model is built around a simple volume balance performed once per month. Conceptually, it follows this sequence:

1. Compute harvested volume from rainfall on the roof.
2. Add harvested volume to starting storage for the month.
3. Subtract household demand for the month.
4. Apply physical limits: storage cannot exceed the tank capacity and cannot drop below zero.

Let the following symbols describe the system:

• A – roof catchment area (m²)
• R – monthly rainfall depth (m)
• C – runoff coefficient (dimensionless, 0–1)
• S_max – cistern storage capacity (m³)
• S_start – storage at the beginning of the month (m³)
• D – water demand for the month (m³)

Monthly harvested volume from the roof is:

The storage update is then:

In words, the planner:

• Starts from the previous month’s storage (S_start).
• Adds harvested water (V = A × R × C).
• Subtracts monthly demand (D).
• Prevents storage from going below zero (a dry tank) or above capacity.

Monthly demand is computed from your inputs for household size and per-person use. If N is the number of people, q is daily use per person, and n_days is the number of days in the month, then:

The emergency buffer you specify (in days of demand) is converted into an equivalent volume. The calculator tracks how often storage dips below this buffer volume even if the tank does not go fully dry, which is useful when you want a safety margin before considering the system “at risk.”

Interpreting your cistern reliability results

Once you run a simulation, the output focuses on how reliably the cistern can supply your assumed demand pattern. Typical summary metrics include:

• Number of dry days: How many days per year the model estimates the tank is empty and cannot meet demand.
• Number of buffer violations: How many days storage falls below your chosen emergency buffer volume, even if not fully empty.
• Minimum storage: The lowest storage level reached during the year, which helps you see the tightest point in the balance.
• Total spillage or overflow: Water that arrived when the tank was already full, highlighting how much potential yield is lost because of limited capacity.

When reading these outputs:

• If dry days are close to zero and buffer violations are rare, your cistern and demand combination is relatively robust under the assumed climate.
• If dry days cluster in specific months, it suggests a seasonal shortfall, often during a pronounced dry season. You might respond by adding storage, reducing demand, or planning a backup supply just for that period.
• If overflow is very high, much of your harvested water has nowhere to go. That can indicate that the tank is smaller than the roof and climate could justify, or that your demand is simply too low to take advantage of all the rainfall.
• If minimum storage sits well above your emergency buffer throughout the year, you may be able to reduce tank size or increase demand while still maintaining your desired level of reliability.

The “Download Water Balance CSV” feature provides month-by-month (or day-equivalent) storage, demand, shortage, and overflow values, which you can analyse in a spreadsheet. This is useful for permit submittals, engineering documentation, or testing multiple design options side by side.

Worked example: off-grid cottage cistern sizing

Consider a family of four living in a 1,900 square foot off-grid cottage with a simple gable roof and a single cistern. They want to decide whether a 2,500 gallon tank is large enough for year-round domestic use in a climate with warm, wet summers and somewhat drier winters.

Inputs

• Unit system: Imperial.
• Roof area: 1,900 ft².
• Runoff coefficient: 0.85 (typical for a metal or well-drained shingle roof with modest losses).
• Cistern capacity: 2,500 gallons.
• Starting fill level: 50 % (1,250 gallons at the beginning of January).
• Household size: 4 people.
• Daily use per person: 35 gallons, for a total of 140 gallons per day.
• Emergency buffer: 3 days of demand (420 gallons).
• Monthly rainfall: Values similar to the defaults in the form, with somewhat wetter summer months.

In January (31 days, 3.1 inches of rain), the model first computes how much rain lands on the roof:

• Rain volume on the roof: 1,900 ft² × 3.1 in ≈ 1,900 × (3.1/12) ft³ ≈ 491 ft³.
• Convert to gallons: 491 ft³ × 7.48 gal/ft³ ≈ 3,670 gallons.
• Apply runoff coefficient (0.85): 3,670 × 0.85 ≈ 3,120 gallons harvested.

January demand is 140 gallons/day × 31 days = 4,340 gallons. Storage evolves as:

• Starting storage: 1,250 gallons.
• After rain: 1,250 + 3,120 = 4,370 gallons, but capped at tank capacity 2,500 gallons, so 1,870 gallons overflow.
• After demand: 2,500 − 4,340 = −1,840 gallons, but storage cannot be negative, so it is set to 0 and 1,840 gallons of demand are not met.

In the simulation, this shortage would be spread across the month as a series of days when the tank runs dry. The exact number depends on when you assume rain arrives within the month, but the planner uses a simple monthly-average approach to estimate an equivalent number of dry days.

Running the same logic for each month shows that, with these inputs, the 2,500 gallon tank experiences repeated shortages in late winter and early spring, even though summer rains mostly refill it. The summary might show a few dozen dry days and regular dips below the 3-day buffer. This suggests that for year-round off-grid use, the cottage either needs a larger cistern, lower daily demand, or a backup source (such as a well or hauled water) in certain months.

Using the example to choose a design

If the family tries a 4,000 gallon tank in the calculator, leaving all other inputs the same, the annual dry days may drop significantly, and buffer violations might disappear. Alternatively, keeping the 2,500 gallon tank but reducing per-person demand to 25 gallons per day (through low-flow fixtures, careful conservation, and perhaps composting toilets) can also improve reliability. The tool lets you quickly run these what-if tests so you can see whether adding storage or cutting use is more effective in your setting.

Comparison of cistern and demand scenarios

Many design questions come down to trade-offs between tank size, household demand, and acceptable risk. The table below summarises how different combinations might behave in a typical moderate-rainfall climate with a 2,000 ft² roof and similar monthly rainfall to the defaults.

From this kind of comparison, you can see patterns:

• Increasing cistern size reduces dry days but eventually reaches diminishing returns if the roof and rainfall cannot fill the tank reliably.
• Reducing daily demand often has a similar effect to adding several hundred gallons of storage, especially in climates with long dry spells.
• Very high overflow can signal an opportunity either to capture more water (with a second tank or additional uses such as irrigation) or to accept a smaller tank if budget is tight and reliability is already high.

Assumptions and limitations of the model

This planner is designed as a transparent, easy-to-use screening tool rather than a full hydrologic or engineering model. Keep the following assumptions and limitations in mind when interpreting the results and using them for design decisions.

• Monthly-average rainfall: The calculator uses average rainfall for each month. It does not model the timing or intensity of individual storms. In reality, a few big storms or a cluster of dry weeks could make reliability better or worse than the monthly-average picture suggests.
• Constant daily demand: Household water use is assumed to be steady from day to day within each month. Seasonal changes in occupancy, holiday visitors, irrigation, or livestock use are not explicitly modelled unless you adjust inputs and rerun scenarios.
• Single, lumped cistern: The model treats storage as one combined tank without accounting for separate compartments, elevation differences, or complex piping that might make some volume unusable.
• Runoff coefficient as a catch-all loss factor: Losses from first-flush diverters, leaks, gutter blockages, evaporation from open tanks, and minor roof wetting are all approximated by the single runoff coefficient. If these losses are large or variable at your site, consider using a more conservative coefficient.
• No water quality or treatment modelling: The planner does not evaluate filtration, disinfection, or water quality, and it does not consider extra water that might be required for filter backwashing or flushing.
• No pump, pressure, or pipe sizing: Mechanical and hydraulic design aspects—such as pump capacity, pressure at fixtures, and friction losses in long pipe runs—are outside the scope of this tool.
• Regulatory and code constraints: Local regulations may limit how harvested rainwater can be used (for example, outdoor irrigation only, or non-potable indoor uses). Always check local codes, standards, and permitting requirements before relying on rainwater for critical uses.
• Representative year only: Because the calculator uses one set of monthly averages, it does not represent multi-year droughts or unusually wet years. For critical systems, it is wise to test more conservative assumptions or obtain long-term climate records.

These limitations mean that the planner is best used for preliminary sizing, scenario comparison, and education about how rainwater harvesting reliability responds to changes in storage, roof area, and demand. For high-stakes projects (such as primary household supply in very dry climates), consult local design standards or a qualified engineer who can account for more detailed climate and system behaviour.

Next steps and further exploration

After you run a few scenarios and review the summary metrics and CSV output, you can:

• Try lower or higher daily demand values to see how conservation affects reliability.
• Adjust the emergency buffer days to reflect your personal risk tolerance or regulatory requirements.
• Experiment with different runoff coefficients to reflect roof upgrades or additional losses (such as more aggressive first-flush diversion).
• Use the CSV export in a spreadsheet to build your own charts of storage through time, or to document design options for clients, permitting agencies, or internal records.

By iterating through these variations, you can move from a rough idea of how much storage you need to a more confident, quantitatively grounded cistern design.