Rainwater Harvesting Calculator

It's a fixed unit conversion, not an estimate: one inch of rain falling over one square foot of horizontal roof area is exactly the volume of a 1-foot-by-1-foot column of water one inch tall, which works out to 0.623 gallons. This factor is the same for every roof and every climate — the calculator multiplies it by your roof area and annual rainfall to get raw catchment volume before any efficiency losses are applied.

Monthly Average simply divides your Annual Collection by 12, spreading it evenly across the year. Real rainfall is almost never even — a location with 35 inches concentrated mostly in winter will collect far more than this average in wet months and far less in dry ones, so use Monthly Average for rough annual planning, not for predicting what a specific month will bring in.

No — it's a worst-case, zero-rain figure. It divides your total tank capacity by your daily usage to show how long the tank alone would last with no further rainfall at all. Any rain that falls during that stretch would extend your actual supply beyond what this number shows, so treat it as a drought floor rather than a realistic day count.

It's the estimated gallons of the very first runoff from each rain event, sized at roughly 0.01 gallons per square foot of roof. That initial flow carries the bulk of accumulated dust, pollen, and bird droppings sitting on the roof between storms, so diverting it before it enters the tank keeps the stored water noticeably cleaner.

It's applied as a direct percentage scale-down on the raw catchment volume (roof area × rainfall × 0.623), representing water lost to wind-driven splash, gutter overflow in heavy storms, and intentional first-flush diversion. Moving the efficiency slider from 80% to 90%, for example, increases every downstream figure — Annual Collection, Monthly Average, and Surplus/Deficit — by that same proportional 12.5% increase.