How this calculator works (monthly water balance)
This tool simulates a simple mass balance for a rainwater tank across 12 months. It is designed for planning and scenario comparison: you can change one input
(tank size, roof area, demand, or rainfall profile) and immediately see how the reliability metrics and monthly table respond.
Each month is treated as a single time step. That means the model does not try to predict the exact day a storm occurs; instead, it totals the month’s rainfall, converts it to
captured volume, and then applies losses and demand. This approach is intentionally transparent and easy to audit.
Monthly simulation steps
- Convert rainfall depth into captured gallons using roof area and runoff efficiency.
- Add captured water to the tank (starting from your initial stored water).
- Apply a monthly storage loss percentage (evaporation/leaks).
- Cap storage at the tank capacity and record any overflow.
- Subtract monthly demand and record any shortage (tank cannot go below zero).
Key formulas and unit assumptions
Captured volume (gallons) is estimated from rainfall depth and roof area:
- Area is converted to square feet internally (if you choose square meters, it is multiplied by 10.7639).
- Rain is monthly rainfall in inches.
- Efficiency is a decimal from 0 to 1 (accounts for losses and imperfect capture).
- 0.623 converts 1 inch of rain over 1 square foot into gallons.
Monthly demand (gallons) is computed from your daily demand and the number of days in the month. If you provide seasonal adjustments, each month is multiplied
by that percentage (e.g., 110 means 10% higher than baseline). February is approximated as 28 days.
How to use the planner
- Enter roof area and choose the correct area unit.
- Set runoff efficiency (typical residential roofs often fall around 0.75–0.95 depending on material and losses).
- Enter tank capacity, initial stored water, and monthly loss percentage.
- Enter average daily demand. Optionally add 12 seasonal adjustment values (comma-separated).
- Select a rainfall profile or choose Custom rainfall and paste 12 monthly rainfall values (inches).
- Click Simulate Reliability to see the summary and monthly table. Use Download Monthly CSV to save results.
Worked example (quick check)
Example inputs: 2,000 sq ft roof, efficiency 0.85, 5,000-gallon tank, 1,000 gallons initial storage, 3% monthly loss, 120 gallons/day demand, and the built-in Seattle rainfall profile.
After simulation you should generally see:
- Higher captured volume in wet months, with possible winter overflow if the tank is small.
- Shortages concentrated in late summer if demand continues but rainfall drops.
- Reliability reported as the percent of months fully supplied, plus overall percent of annual demand met.
If results look unrealistic, the most common causes are: entering efficiency as a percent (e.g., 85 instead of 0.85), mixing up inches vs. millimeters, or pasting fewer/more than 12 rainfall values.
Limitations and assumptions
- Monthly time step: rainfall is treated as a monthly lump sum; storm timing within the month is not modeled.
- First-flush and treatment: the model does not subtract first-flush diversion or account for water quality constraints.
- Loss model: storage loss is applied as a simple percentage; real losses vary with temperature, tank design, and maintenance.
- Climate variability: built-in profiles are representative and may not match your exact site or future conditions.
Planning for dependable rainwater supply
Rainwater harvesting can reduce municipal demand, provide backup supply during restrictions, and improve site resilience. The practical design question is not just “How much can I collect in a year?”
but “When will I have water available, and how often will I run short?” Seasonality matters: a location with high annual rainfall can still experience summer shortages if most rain falls
in winter, while an arid location may see brief refill periods followed by long dry stretches.
This planner is designed for early-stage sizing and scenario comparison. It helps you explore tradeoffs between roof area, runoff efficiency, tank size, and demand. For example, if your results show
frequent overflow in wet months, you may benefit more from additional storage (or additional uses for rainwater) than from increasing catchment area. If results show persistent shortages even with a large
tank, the limiting factor is likely rainfall volume or demand level rather than storage.
What the reliability metrics mean
- Monthly reliability (% of months fully supplied): the share of months where the tank could meet the entire modeled demand.
- Demand coverage (% of annual demand met): total gallons supplied divided by total gallons demanded across the year.
- Overflow: water that could have been captured but was lost because the tank was full after losses were applied.
- Shortage: unmet demand in months where the tank ran out.
Tips for better inputs
Use measured roof area when possible, and choose an efficiency that reflects your system (gutters, screens, first-flush diversion, and roof material). For demand, start with the specific end uses you plan
to supply. If you are modeling irrigation, seasonal adjustments are often more realistic than a flat daily demand.
Using custom rainfall
If you have local climate normals or a rain gauge record, paste 12 monthly totals (in inches) in order from January to December. The calculator requires exactly 12 values to keep the simulation aligned with
the monthly table and CSV export.
Detailed guidance: choosing realistic values
The quality of a reliability estimate depends on the quality of the inputs. The model is straightforward, but real systems vary widely. Use the notes below to pick values that match your site and to avoid
the most common planning mistakes.
Roof catchment area
Roof area should represent the portion of the roof that actually drains to your collection system. If only one side of a gable roof is connected to gutters that feed the cistern, use that portion rather than
the entire building footprint. For complex roofs, it can be helpful to break the roof into rectangles and triangles and sum the areas. When in doubt, a conservative approach is to slightly under-estimate area;
you can always run a second scenario with a higher value to see sensitivity.
Runoff efficiency (0–1)
Efficiency captures losses between rainfall and stored water. Losses can include wetting of the roof surface, splash-out, gutter overflow during intense storms, debris screens, and intentional first-flush diversion.
Smooth, clean roofs with well-sized gutters may be near the high end of the range, while rough surfaces or systems with frequent clogging may be lower. If you are unsure, run at least two scenarios (for example,
0.75 and 0.90) and compare reliability.
Storage capacity and initial storage
Storage capacity is the maximum usable volume of the tank. Some tanks have a “dead storage” zone below the outlet or above a sediment layer; if that volume is not usable, subtract it from the nominal tank size.
Initial storage is the amount of water in the tank at the start of the year. If you are planning a new installation, you might set initial storage to zero to represent an empty tank at commissioning. If you are
evaluating an existing system, choose a realistic starting level for the season you care about.
Monthly storage loss (%)
The loss percentage is a simplified way to represent evaporation, leaks, and operational losses. Covered tanks in mild climates may have very low evaporation, while open or partially open storage can lose more.
Leaks, overflows from poorly sealed fittings, and routine maintenance drains can also contribute. Because the model applies the loss as a percentage, the absolute gallons lost will be larger when the tank is fuller.
If you do not have measured data, start with a small value (1–3%) and test a higher value (5–10%) to see how sensitive your reliability is.
Demand: what to include
Demand should include only the uses you intend to supply with rainwater. Many households start with outdoor irrigation, then expand to non-potable indoor uses such as toilet flushing or laundry. If you include all
household water use, the model may show low reliability even though the system could still be valuable for a subset of uses. A practical workflow is to model a “core” demand (critical uses you want to cover) and a
“stretch” demand (nice-to-have uses) as separate scenarios.
Seasonal demand adjustments (12 values)
Seasonal adjustments let you represent predictable changes such as summer irrigation, winter dormancy, or occupancy changes. Enter 12 comma-separated percentages in order from January to December. For example, a
garden-heavy home might use: 80,80,90,100,120,140,150,150,130,110,90,80. The model multiplies each month’s baseline demand by the corresponding percentage divided by 100.
Rainfall profiles and custom rainfall
Built-in profiles are representative monthly totals for several U.S. climates. They are useful for learning how seasonality affects storage, but they may not match your neighborhood, elevation, or microclimate.
For design work, consider using local climate normals (often 30-year averages) or your own rain gauge record. If you paste custom rainfall, ensure you provide exactly 12 values and keep units in inches. If your
source data is in millimeters, convert to inches first (1 inch = 25.4 mm).
Interpreting results: what to do with reliability, overflow, and shortage
The summary panel reports several metrics that answer different planning questions. Monthly reliability tells you how often the system fully meets demand in a month; it is a strict measure that treats
a month with even a small shortage as “not met.” Demand coverage tells you what fraction of total annual demand is supplied; it can be high even if a few months have shortages. Overflow
indicates wasted potential capture due to limited storage, while shortage indicates unmet demand.
Use these metrics together. If overflow is high and shortage is low, the system is storage-limited and a larger tank (or additional uses during wet months) may improve performance. If shortage is high and overflow
is near zero, the system is rainfall-limited or demand is too high; reducing demand, increasing catchment area, or adding an alternative supply may be more effective than increasing storage.
Scenario planning checklist
- Baseline: enter your best estimates for area, efficiency, demand, and rainfall.
- Conservative: reduce efficiency and rainfall slightly, and increase demand slightly.
- Aggressive: increase efficiency and rainfall slightly, and reduce demand slightly.
- Compare: look for inputs that change reliability the most; those are the best targets for measurement or design improvements.
- Document: download the CSV for each scenario so you can compare month-by-month behavior.
Frequently asked questions (practical)
Why does the model show shortages even in a rainy city?
Many climates have rainfall concentrated in certain seasons. If demand peaks in the dry season (for example, summer irrigation), a small tank may not carry enough water from wet months into dry months. In that case,
you may see both winter overflow and late-summer shortages. That pattern is a strong signal that storage size and demand timing matter as much as annual rainfall.
Why is February treated as 28 days?
The simulation uses a simple fixed calendar to keep the model lightweight and predictable. February is approximated as 28 days, which is close enough for planning-level estimates. If you need higher precision,
you can adjust daily demand slightly or use a more detailed daily model.
Can I use this for potable water design?
The calculator estimates quantity and timing, not water quality. Potable use typically requires treatment and may be regulated. Use this tool to understand storage and reliability, then consult local codes and
qualified professionals for treatment, backflow prevention, and permitting requirements.
What if my roof area is in square meters and rainfall is in millimeters?
You can enter roof area in square meters using the Area Unit selector. Rainfall must be entered in inches for custom profiles. Convert millimeters to inches by dividing by 25.4. Keeping units consistent is one of
the most important steps for a trustworthy result.
Assumptions recap (for reporting)
This planner assumes: (1) monthly rainfall totals are applied as a lump sum; (2) captured volume is proportional to roof area, rainfall depth, and runoff efficiency; (3) storage losses are a fixed percentage applied
monthly; (4) storage cannot exceed tank capacity and cannot go below zero; and (5) demand is applied monthly based on daily demand and optional seasonal multipliers. These assumptions make the model easy to explain
and reproduce, which is useful for early planning, education, and quick comparisons.