How this calculator works (model overview)
The model treats your cistern like a simple bank account. Each month, rainfall adds water (inflow) and household use removes water (demand).
Storage is never allowed to exceed the tank capacity (excess becomes overflow) and never allowed to go below zero (unmet demand becomes shortfall).
Because the simulation is monthly, it is best for planning and comparison of scenarios rather than predicting day-by-day reliability.
The safe-yield number is computed by searching for the largest constant daily demand that produces no shortfall in the modeled year.
Internally, the script uses a binary search: it tries a daily demand, runs the 12-month simulation, and then adjusts the demand up or down until it finds the highest value that still keeps annual shortfall essentially at zero.
This approach is fast and stable for a single-page calculator, and it makes it easy to compare “what if” changes (bigger tank, more roof area, lower first flush, or a different rainfall pattern).
Units, conversions, and key formula
This page uses imperial units: rainfall in inches, roof area in square feet, and volumes in gallons.
A common conversion used in rainwater harvesting is:
1 inch of rain on 1 ft² ≈ 0.623 gallons.
That constant is built into the simulation.
For each month m:
- Gross capture (gal) = Rainfall(in) × Roof area(ft²) × 0.623 × Efficiency
- First-flush loss (gal) = First flush (gal/event) × Rain events in month
- Net inflow (gal) = max(0, Gross capture − First-flush loss)
- Monthly demand (gal) = (Average daily demand) × (days in month)
Important: the collection efficiency input in the form is a percent (e.g., 85%), but the simulation expects a fraction (0.85).
This page converts the percent to a fraction automatically so the math matches the labels.
Introduction: Assumptions and limitations (what’s included and what isn’t)
- Monthly time step: rainfall is aggregated by month; storm timing within a month is not modeled.
- First flush by event count: losses are estimated as a fixed gallons/event × events/month.
- No explicit evaporation/leaks: include these in efficiency or demand if they matter for your system.
- Water quality and treatment: not modeled; follow local codes for potable systems.
- Planning use: use long-term normals for typical planning, and also test a dry-year scenario for resilience.
If you are designing for critical uses (for example, a remote property with no backup supply, or a facility that must maintain water for sanitation), treat the safe-yield output as a baseline.
Add margin by lowering rainfall inputs, increasing demand inputs, or increasing the starting storage requirement in your own planning.
Many users run at least three cases: a typical year, a dry year, and a “future demand” year.
How to use: Worked example (using the default values)
Suppose you have a 2,400 ft² roof, 85% collection efficiency, a 5,000-gallon cistern, and you start the year half full.
You enter monthly rainfall totals (inches) and a monthly demand profile (gallons/month). You also assume a 15-gallon first flush per storm and provide the number of storms per month.
After running the simulation, the summary table will show the estimated safe yield (gal/day) and the annual totals.
The monthly table then helps you diagnose when the system is stressed.
For example, if the minimum end-of-month storage occurs in August, that indicates late-summer demand and low rainfall are the limiting factors.
A quick sanity check is to compare annual net inflow to annual demand. If annual inflow is far below annual demand, the system cannot meet the load without a backup supply.
If annual inflow is above annual demand but you still see shortfall, the issue is usually seasonality + limited storage:
wet months can’t fully “carry” you through dry months unless the tank is large enough.
Practical tips for better inputs
- Roof area: use the projected catchment area that drains to your collection point(s), not the home’s floor area.
- Efficiency: typical planning values are 75–90% depending on roof material, gutters, filters, and maintenance.
- Rainfall: use 12 values (Jan–Dec). If you only have annual rainfall, divide into a monthly pattern from local normals.
- Demand: if you only know average daily use, convert to monthly totals (daily × days/month) and enter 12 values.
- First flush: set to 0 if you do not divert first flush; otherwise use your diverter’s actual volume.
- Events per month: count only runoff-producing storms. Drizzle that never reaches gutters should not be counted as an event.
How to interpret results (what to look for first)
The results are most useful when you read them in a specific order. Start with the summary metrics, then use the monthly table to understand the “why.”
In practice, most design decisions come down to which constraint is binding: catchment, storage, or demand.
- Annual shortfall: if this is greater than zero, the modeled system cannot meet the tested demand profile.
- Minimum end-of-month storage: this shows how close you get to empty. A value near zero means the system is fragile to small droughts.
- Annual overflow: high overflow means you are losing water because the tank is full during wet months. That can indicate a storage bottleneck.
- Safe yield (gal/day): use this as a planning target for steady demand. Compare it to your intended uses (toilets, laundry, irrigation).
If you are using rainwater for irrigation, consider that irrigation demand is often seasonal and can be reduced during drought.
A common strategy is to plan for a “core” indoor demand that is always met, and treat outdoor use as flexible.
You can model that by lowering summer monthly demand values or by running two scenarios: one with irrigation included and one without.
Resilience checks (dry-year and growth scenarios)
Planning improves when you test sensitivity. Two simple checks catch most surprises:
- Dry-year check: reduce each monthly rainfall value by 10–30% and rerun. If safe yield collapses, your system is rainfall-limited and needs more storage margin or backup supply.
- Growth check: increase monthly demand to reflect additional occupants, a new bathroom, or expanded irrigation. If shortfall appears, demand management or system expansion is needed.
You can also test infrastructure changes. Increasing roof area (adding a carport, garage, or shed roof) increases capture in every month.
Increasing tank capacity reduces overflow and can improve dry-season reliability, but only up to the point where capture becomes the limiting factor.
Reducing first-flush volume (or improving filtration so you can divert less) increases net inflow, especially in months with many small storms.
Planning notes: using safe yield to size a cistern
Safe yield is useful because it separates two questions that are often confused: “How much rain falls in a year?” and “How much water can I reliably use?”
Annual totals can look generous while the tank still runs dry in late summer, because the tank can only store a limited amount of water between storms.
A monthly balance highlights the seasonal pinch points and shows whether you need more storage, more catchment area, or lower demand.
When you review results, focus on three diagnostics:
(1) shortfall (any unmet demand), (2) minimum end-of-month storage (how close you get to empty), and (3) overflow (how much water you lose because the tank is full).
High overflow with no shortfall often means you could increase demand (or add storage) without risk; shortfall with low overflow usually means you are capture-limited or demand is too high for the dry season.
If you want a more conservative plan, rerun the calculator with rainfall reduced (for example, multiply each monthly rainfall value by 0.75 for a 25% drier year).
If the safe yield drops sharply, storage and seasonality are driving reliability.
Common use cases (and how to model them)
Different households use rainwater in different ways, and the best way to use this planner is to match the inputs to your intended use.
Below are common scenarios and what to enter.
- Indoor non-potable (toilets + laundry): demand is relatively steady year-round. Use a fairly flat monthly demand list.
- Outdoor irrigation: demand is seasonal. Increase summer demand values and reduce winter values to reflect actual watering schedules.
- Whole-house supply: demand is higher and reliability expectations are stricter. Run a dry-year rainfall case and consider increasing storage.
- Backup/emergency supply: set demand to the minimum you must maintain, then check minimum storage and shortfall under drought.
How to sanity-check your inputs before trusting the output
A quick “back of the envelope” check helps confirm you entered reasonable values. For example, if your annual rainfall is about 36 inches and your roof is 2,400 ft²,
the gross annual capture at 100% efficiency is roughly 36 × 2,400 × 0.623 ≈ 53,800 gallons.
At 85% efficiency that becomes about 45,700 gallons before first-flush losses.
If your monthly demand totals 60,000 gallons, the system will not meet that demand without supplemental water.
If your demand totals 20,000 gallons, the system may be reliable even with a modest tank.
Also check the event assumptions. If you enter 15 gallons per event and 7 events in a month, that is 105 gallons of first flush.
In a month with only 2 inches of rain, first flush can be a meaningful fraction of capture, especially for small roofs.
If your first-flush device is smaller (or you only divert first flush after long dry periods), consider lowering the value.
What to do with overflow (designing for beneficial use)
Overflow is not necessarily “bad.” It can indicate that your tank is doing its job and that your roof produces more water than you can store.
If overflow is large, you have options: increase storage, increase demand (for example, add irrigation during wet months), or route overflow to a rain garden, swale, or infiltration trench.
In many climates, capturing the last few thousand gallons of overflow requires disproportionately large tanks, so it can be more cost-effective to manage overflow on-site.
About the drought buffer input
The form includes a drought buffer in days of demand because many real systems reserve water for a minimum service level.
The current simulation displays the input for planning but does not enforce it as a constraint.
If you want to approximate a buffer, you can reduce your effective tank capacity by reserving a portion of storage (for example, treat a 5,000-gallon tank as 4,000 gallons usable)
or reduce the safe-yield target until the minimum end-of-month storage stays above your desired reserve.
