Rainfall Volume Calculator (Roof & Land Harvest)

Annual rainfall × footprint × ~0.85: a modest 120 m² roof under 800 mm of annual rain grosses 96,000 L — captures roughly 80,000 L, about 220 L/day averaged. In US units, a 1,300 ft² roof in a 32-inch-rain climate banks ~22,000 gallons a year. The catch is timing: most climates deliver that in a wet season, so tank sizing (not roof area) usually limits how much of it you keep — size tanks against your longest dry spell, not the annual total.

Smooth metal roofs 90–95%; tile and shingle 80–90% (absorption and texture); flat gravel roofs 70–80%. Subtract first-flush diversion (the first 0.5–1 mm sent to waste to carry off dust and droppings — disproportionately costly in light-shower climates), gutter overshoot in downpours, and evaporation from hot surfaces in light rain. Bare land is another world entirely: infiltration eats most of it — runoff coefficients run 0.1–0.3 for soil, which is why farm-pond harvesting needs hectares, not roofs.

It's 623 gallons per 1000 ft² — and meteorologically a decent soaking: light rain accumulates under 0.1 in/hr, moderate 0.1–0.3, heavy beyond that; an inch in an hour is a downpour that tests gutters (a 2000 ft² roof sheds 1,250 gallons in that hour — over 20 gal/min through the downspouts). Design plumbing for rainfall RATE (your local 10-year storm intensity), and tanks for rainfall DEPTH over the season; the two sizing problems share this page's arithmetic at different time scales.

Audit in this order: gutters overflowing in bursts (undersized or clogged — the biggest silent loss), first-flush diverter volume on every event (20 L × 60 storms is 1,200 L/year), wind-blown rain missing the roof plane in storms, evaporation in light sprinkles that wet the roof but never run, and depth measured at an airport miles away (convective rain varies 2× across a town). A $10 rain gauge on-site usually reconciles half the discrepancy immediately.