Rainwater Harvesting System Design and Cistern Sizing

Collection Area and Annual Yield

The fundamental collection equation:

Volume (gallons) = Rainfall (inches) × Collection Area (ft²) × 0.623 × Efficiency

Where 0.623 converts inch-ft² to gallons (1 inch of rain on 1 ft² = 0.623 gallons).

Collection efficiency accounts for losses from evaporation, splash, gutter overflow, first-flush diversion, and surface absorption. Typical values:

• Metal roofing: 0.90–0.95
• Asphalt shingles: 0.80–0.85
• Concrete tile: 0.75–0.85
• Clay tile: 0.75–0.80
• Green (vegetated) roof: 0.20–0.40

Example: A 2,000 ft² metal roof in an area receiving 40 inches of annual rainfall:

Annual Yield = 40 × 2,000 × 0.623 × 0.90 = 44,856 gallons

That is roughly 3,738 gallons per month on average -- but rainfall is not uniform. The monthly distribution matters enormously for cistern sizing. A region with 40 inches distributed evenly (3.3"/month) requires a much smaller cistern than one with 40 inches concentrated in 6 wet months and 6 dry months.

Tank Storage Planning: Supply-Demand Balance

Storage planning is a mass balance prompt. The storage row must be checked against local rainfall, dry spells, seasonal demand, tank product data, dead storage, overflow routing, and code or health requirements. A month-by-month calculation can organize the arithmetic:

For each month, calculate:

Supply = Monthly Rainfall × Collection Area × 0.623 × Efficiency
Demand = Monthly water usage from the harvested supply
Net = Supply − Demand
Running Storage = Previous month's storage + Net (clamped at 0 and the tested storage capacity)

This is still a planning prompt. A selected tank needs product listing, structural/foundation review, access, venting, overflow, freeze protection, maintenance, mosquito control, pump and controls review, and local code/AHJ acceptance where applicable.

Quick-estimate methods can be useful for early review, but they do not replace a local hydrology record, actual demand history, treatment requirements, or qualified design review.

First Flush Diversion and Water Quality

The first rain after a dry spell can wash dirt, bird droppings, pollen, debris, roofing chemicals, and other contaminants off the collection surface. A first-flush prompt can help organize local water-quality review, but the needed diversion volume depends on roof condition, antecedent dry period, tree cover, bird activity, intended use, and local health or plumbing requirements.

A common planning row is 1 gallon per 100 ft² of collection area. A 2,000 ft² roof would screen at 20 gallons. Do not treat that as proof that the water is acceptable for any use.

Beyond first-flush diversion, water-quality requirements depend on the intended use, current local rules, component listings, treatment system, testing records, cross-connection control, and qualified review. Potable, cooking, bathing, laundry, and toilet-flushing uses can each trigger different health, plumbing, signage, treatment, and inspection requirements.

Pump Selection and Distribution

If the tank is above the point of use, gravity head can be estimated at about 0.433 psi per foot of water elevation. Most pressurized uses still need product-specific pump, pressure tank, piping, filtration, controls, and electrical review.

The calculator can estimate a flow figure from daily demand and assumed pump run hours, but it does not select a pump, pressure tank, pipe size, backflow assembly, filter, treatment system, or control package. Indoor use also needs adopted plumbing code, cross-connection control, signage, inspections, and AHJ review.

Freeze-prone climates, buried tanks, exterior piping, pump vaults, heat trace, maintenance access, overflow routing, and electrical work all need product instructions and qualified review.