How it works
One inch of rain falling on one square foot of roof is 144 cubic inches of water, or 0.623 gallons. Multiply by the roof's footprint area and the rainfall, then by a collection efficiency to allow for losses, and you get gallons collected.
Use the roof's horizontal footprint, including overhangs, not the sloped surface: rain falls vertically, so a steep roof catches no more water than a flat one with the same footprint. If only some downspouts feed your barrels, use just the roof area that drains to them.
Efficiency of 75% to 90% is typical. Some water splashes over gutters, evaporates or is diverted by a first-flush device that sends the dirtiest initial runoff away. A 1,500 sq ft roof collects about 750 gallons from a single inch of rain at 80%, far more than one 55 gallon barrel can hold.
Formula
Gallons = roof footprint (sq ft) × rainfall (in) × 0.623 × efficiency. Metric: liters = area (m²) × rainfall (mm) × efficiency.
Questions people ask
How many gallons does 1 inch of rain produce from a roof?
About 623 gallons per 1,000 sq ft of roof footprint at 100% efficiency, or roughly 500 gallons at a realistic 80%.
Does roof pitch change how much water I collect?
No. Collection depends on the horizontal area the roof covers, because rain falls roughly vertically. Pitch affects how fast water runs off, not how much.
How many rain barrels do I need?
Divide the gallons from a typical storm by the barrel size. A single downspout draining 400 sq ft of roof fills a 55 gallon barrel with about a quarter inch of rain.
Is rainwater harvesting legal?
It is legal in most US states, including Michigan, for uses like garden irrigation. A few states regulate volumes or require permits for larger systems, and drinking water use needs treatment.