Rainwater Harvesting Tank Sizer Calculator

Purpose and model boundary

Runs a twelve-month storage balance across one entered tank or a fixed 21-capacity ladder and exposes reliability, overflow, reserve, and simple economics.

Do not treat this result as a hydraulic or plumbing design, flood-protection guarantee, code-compliance determination, permit approval, equipment selection, safety certification, cost quote, or endorsement by a cited source. Site geometry, rainfall, runoff, soil, pipe, pump, tank, irrigation, manufacturer, locally adopted code, and approval inputs remain the user's responsibility.

Inputs and units

• yield basis: a listed route or text value.
• demand basis: a listed route or text value.
• tank mode: a listed route or text value.
• initial storage mode: a listed route or text value.
• roof area ft2: entered in ft^2.
• runoff coefficient: entered in fraction.
• collection efficiency: entered in fraction.
• first flush depth in: entered in inches per month.
• daily demand gal: entered in gal/day.
• entered tank capacity gal: entered in gal.
• minimum candidate capacity gal: entered in gal.
• maximum candidate capacity gal: entered in gal.
• target reliability: entered in fraction.
• minimum ending reserve fraction: entered in fraction of capacity.
• entered initial storage gal: entered in gal.
• fixed system cost: entered in user currency.
• tank cost per gal: entered in user currency/gal.
• water value per 1000 gal: entered in user currency/1,000 gal.

The monthly grid contains 12 fixed data rows plus one header row with these columns: Month label; fixed/read-only, Days (days), Rainfall (in/month), Direct captured inflow (gal/month), Entered demand (gal/month), Daily-demand multiplier (fraction). Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Monthly captured inflow is entered directly or calculated as:

Y_m = max(0,R_m-F) A C eta 0.623

Y_m is in gal/month, R_m is monthly rainfall (in), F is monthly first-flush depth (in), A is connected roof area (ft^2), C is runoff coefficient, eta is collection efficiency, and 0.623 converts one inch over one square foot to gallons. Monthly demand is entered directly or calculated as:

D_m = d_base n_m k_m

d_base is gal/day, n_m is days, and k_m is the row multiplier.

For tank capacity K, each month applies:

available_m = S_(m-1)+Y_m

shortfall_m = max(0,D_m-available_m)

overflow_m = max(0,available_m-D_m-K)

S_m = min(K,max(0,available_m-D_m))

Every volume is in gallons. Volumetric reliability is:

Rel = 1 - sum(shortfall_m)/sum(D_m)

or 1 when annual demand is zero.

Evaluate mode runs one entered K; select mode runs 21 evenly spaced capacities from the entered minimum through maximum and selects the first for which Rel meets target and ending storage is at least K times the reserve fraction. Annual delivered water is annual demand less shortfall, annual value is delivered gallons divided by 1,000 times entered water value, installed cost is fixed cost plus K times cost/gal, and simple payback is cost divided by positive annual value.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• selected tank capacity gal (gal): Entered capacity or first passing fixed-ladder capacity.
• annual delivered water gal (gal/year): Demand less selected-capacity shortfall.
• annual shortfall gal (gal/year): Demand unmet by monthly stored water.
• volumetric reliability (fraction): One minus annual shortfall divided by annual demand.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• All shipped default rainfall, demand, cost, and monthly-profile values are synthetic illustrative demonstrations.

• Rainfall, demand, costs, and monthly profiles are synthetic editable examples.

• One aggregate tank and one monthly time step are used.

• Roof yield uses 0.623 gallons per inch-foot squared before entered coefficients.

• Monthly aggregation can hide storm timing, drawdown, drought sequences, first-flush event frequency, water-quality constraints, freeze protection, pump energy, and overflow routing.

• Verify potable/nonpotable rules, mosquito control, structural support, backflow protection, treatment, setbacks, and permits locally.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 135 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• EPA Green Infrastructure - Primary public context for rainwater capture and runoff management. Official U.S. government source; no local regulation or dataset is embedded.
• NOAA Atlas 14 precipitation frequency estimates - Primary public reminder that local precipitation inputs require location-specific review. Official U.S. government source; no NOAA rainfall table is embedded.
• NIST volume units - Primary unit-conversion provenance for gallons and cubic volume. Official public U.S. government reference; conversions are independently implemented and no source material is copied.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, forecast, or external dataset is embedded. The calculator independently implements general hydraulic, water-balance, and home-project engineering arithmetic from cited public and official sources; all defaults, labels, conductor/load rows, equipment assumptions, diagrams, and test values are synthetic illustrations.