The 0.623 rainwater formula, explained
One inch of rain falling on one square foot deposits exactly 1/12 of a cubic foot of water. A cubic foot holds 7.48 US gallons, so 7.48 ÷ 12 = 0.623 gallons per square foot per inch of rain. That constant is the whole secret of every rain harvesting calculator:
gallons = roof sq ft × rain inches × 0.623 × efficiency
Two details people get wrong. First, the area is the roof’s horizontal footprint (the shadow it casts at noon), not the sloped surface area — rain falls vertically, so pitch does not add catchment. Second, no system captures everything: evaporation, splash-out, gutter overflow and absorption by the roofing material all take a cut, which is what the efficiency factor represents.
Roof material efficiency
Typical collection efficiencies, aggregated from public extension-service guidance:
| Roof type | Typical efficiency | Notes |
|---|---|---|
| Metal (standing seam, ribbed) | 0.90–0.95 | Smooth and non-porous; best for potable-intent systems |
| Asphalt shingle | 0.85–0.90 | Slightly porous; granules absorb a little; fine for irrigation |
| Tile (clay, concrete) | 0.80–0.90 | Porous tiles absorb more in light rain |
| Flat / built-up | 0.70–0.80 | Ponding and slow drainage lose more to evaporation |
| Green roof | 0.20–0.50 | By design — the planting retains most rainfall |
These are planning bands, not lab figures. In a light drizzle the roof itself absorbs proportionally more; in a cloudburst the gutters become the bottleneck and overflow. Annual averages tend to land near the middle of each band.
What size rain barrel do I need?
Work backwards from the roof area feeding the barrel. A standard 55-gallon barrel under a downspout draining 400 sq ft of shingle roof fills with just 0.25 inches of rain (400 × 0.25 × 0.623 × 0.9 ≈ 56 gallons). That is the most common surprise in rain barrel ownership: in most climates a single barrel fills in the first minutes of a real storm and overflows for the rest of it.
The barrel module above shows both the gallons your chosen rainfall delivers to the barrel and the inches of rain needed to fill it. If the fill figure is a small fraction of a typical storm, either daisy-chain barrels, route the overflow somewhere deliberate (a drain or rain garden), or step up to a cistern.
Storage tank sizing
Tank sizing is a balance between three numbers: how much you can capture per period, how much you use per day, and how many days of storage you want to ride out between rains. The tank module divides your tank size by daily demand to show days of supply, and compares your capture against demand so you can see whether the limiting factor is the roof or the tank.
A practical sequence: enter your driest relevant month’s rainfall in the main calculator, compare the captured gallons against that month’s demand (daily use × 30), and size the tank to bridge the gap. Oversizing a tank beyond what the roof can refill just stores air.
First-flush diversion
The first water off a roof carries the accumulated dust, pollen, droppings and debris since the last rain. A first-flush diverter dumps that initial slug — a common generic guideline is to divert roughly 1–2 gallons per 100 sq ft of roof — before clean water flows to storage. On a 1,500 sq ft roof that is 15–30 gallons sacrificed per storm, a worthwhile trade for irrigation water quality and near-essential if the water touches edible plants. Subtract it mentally from small-storm yields: a 0.1-inch shower may produce nothing but flush.
Metric version
The metric formula is cleaner because the units already agree:
liters = roof area (m²) × rainfall (mm) × efficiency
One millimetre of rain on one square metre is exactly one liter; multiply by ~0.9 for a typical roof and you are done. Switch the calculator to metric and every field and result converts — no separate tool needed.