The roof's footprint (length × width), not the sloped surface area.
Inches per year. Use the state autofill if you're not sure.
Optional — sets the rainfall field to that state's average.
Usually 75–90% after gutter losses, first-flush diversion, and evaporation.
A roof collects roughly 0.623 gallons per square foot per inch of rain. The calculator multiplies your roof footprint by your annual rainfall and that factor, then applies your collection efficiency. Picking a state fills in that state's average rainfall from our state data, but a local figure is always better if you have one.
Capture is only half the planning — size your storage for how you'll actually use the water (garden irrigation, livestock, or household) and for the dry stretches between rains. For the legal side of collecting and using water, see our guide to water rights for homesteaders.
For every inch of rain, a roof collects about 0.623 gallons per square foot of footprint. Multiply your roof's footprint by your annual rainfall and that factor, then apply a collection efficiency of roughly 75–90% to account for gutter losses, first-flush diversion, and evaporation.
Use the footprint — the flat area the roof covers when viewed from above (length × width). A pitched roof doesn't collect more water than its footprint, because rain falls vertically.
75–90% is realistic for a well-built system. Gutters, downspout screens, first-flush diverters, splashover in heavy storms, and evaporation all reduce yield below the theoretical maximum.
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