Sizing a rainwater harvesting system
How catchment area, runoff coefficient, first flush and tank size decide how much roof rainwater you can actually store and use.
Catchment area and the collection factor
The catchment area is the flat footprint of the roof measured from above, not the sloped surface you walk on. Rain falls vertically, so a pitched roof and a flat roof with the same footprint collect the same volume of water. Each square foot yields about 0.623 gallons for every inch of rain, which is where the calculator's constant comes from. Multiply footprint by rainfall by that constant to get the theoretical maximum before any losses. Gutters, downpipes and the tank inlet then decide how much of that maximum you actually keep.
The runoff coefficient by roof type
Not every drop that lands on the roof reaches the tank. The runoff coefficient, entered here as collection efficiency, captures splash, evaporation, wetting of the surface and overflow during intense bursts. Smooth sheet metal roofs run high, often 0.9 or better, while rough tiles, gravel or green roofs lose more and sit lower. A common planning range is 0.75 to 0.9. Pick a value near the low end if your gutters are shallow or your climate brings short, heavy storms.
First flush and water quality
The first water off a roof carries the most dirt: dust, pollen, leaf litter and bird droppings that built up between rains. A first flush diverter sends that initial volume to waste before the cleaner water reaches the tank. Diverting the first fraction protects water quality but lowers yield, which is one reason real efficiency sits below one. Even after diversion, roof water is not drinking water without filtration and disinfection. It suits irrigation, toilet flushing, laundry and other non-potable uses.
Sizing the storage tank
A tank should balance how fast it fills against how fast you draw it down. Size the inflow from your roof area, a typical local storm and your chosen efficiency, then compare it to the days between rains and your daily demand. An oversized tank wastes money and can let water stagnate, while an undersized one overflows and spills the surplus you paid to collect. Many households target enough storage to bridge the longest common dry spell for their main use. Rainfall records for your area make this estimate far more reliable than a single storm figure.