Rainwater Harvesting Explained: The Math of Free Water

Every rainstorm is a delivery your roof already receives — the only question is how much of it you keep. The arithmetic is genuinely pleasant: one millimeter of rain on one square meter of roof is, by definition of the units, exactly one liter of water. Everything else in rainwater harvesting is a refinement of that one line.

The one-line harvest formula

Capture = roof area × rainfall depth × a runoff coefficient. In metric units, liters = mm × m² before any losses: a 100 m² roof under a 10 mm shower receives 1,000 liters. The metric system's base units make depth-times-area come out directly in volume — no conversion factors, no memorization.

Imperial units need one constant: one inch of rain on one square foot yields 144 cubic inches, and with 231 cubic inches to the gallon that is 0.623 gallons per square foot per inch. A 1,000 ft² roof under one inch of rain collects 623 gallons before losses. Multiply through for any combination — the formula is perfectly linear, which is why the same calculation answers "this storm" and "this year" with identical machinery.

The honest part: the runoff coefficient

The theoretical maximum is a lie your gutters tell. Splash-out at roof edges, film left wetting the surface, wind-driven misses and first-flush debris all conspire, so only a fraction of the falling rain becomes usable water. That fraction is the runoff coefficient, and it varies more than most people expect by surface:

Metal sheeting sheds about 95% — smooth, impermeable, beading fast. Clay or concrete tile and poured concrete run near 90%. Asphalt shingle lands around 85%, with texture and granule roughness holding water back. Gravel roofs come in near 80% as the stones trap and slow sheet flow. Green (planted) roofs are the outlier at roughly 55%: the soil and root layer is designed to absorb rain, which is the point, but it halves your harvest.

Applying the coefficient honestly changes decisions. That 1,000 ft² shingle roof yields 530 gallons per inch of rain, not 623 — and sizing a tank on the theoretical number means buying 15% more tank than your roof can fill.

From storm to season

Because the math is linear, annual planning is the same multiplication with a bigger rainfall number. A 150 m² roof in a climate that delivers 900 mm a year sees about 135,000 liters fall on it; at a metal roof's 95% efficiency, roughly 128,000 liters is harvestable. That is enough to irrigate a serious garden, feed toilets and laundry, or make a dent in household demand — the reason rain harvesting is often the cheapest water source available for non-potable uses.

Seasonality matters as much as the total, though. A Mediterranean climate that delivers 500 mm almost entirely between November and March produces a huge winter yield and a bone-dry summer — storage, not catchment, becomes the constraint. Conversely, frequent light rain in a modest-total climate keeps tanks cycling year-round. Run the calculator on a representative single storm to size a first tank, then on monthly totals to see the storage rhythm.

Tank sizing and first flush

A common starting anchor: size storage to absorb one typical storm without overflowing. In many temperate climates that is 25–50 mm of rain; a 200 m² tiled roof under 30 mm yields about 5,400 liters, which is why 5,000-liter tanks are such a common first purchase. From there, grow storage toward capturing shoulder-season rain if your demand profile justifies it.

Plumbing matters too: the first minutes of any storm rinse dust, leaves and bird droppings off the roof, which is why serious systems divert that first flush away from storage. With basic filtering, harvested rain is excellent for irrigation, toilet flushing and laundry; making it potable requires first-flush diversion, filtration and disinfection under local regulations — a different tier of project entirely.

Try it yourself

The Rainwater Harvesting Calculator applies the full formula — area, rainfall, surface coefficient — and shows liters, US gallons and cubic meters, plus the theoretical capture and the liters lost to surface inefficiency. To size the roof from its footprint, use the Area Converter; to see what the harvested volume looks like in tank dimensions, the Volume Converter converts liters into the units your tank brochures use.