Roof Drainage Calculator

Runoff Gallons, Peak Flow, Downspouts and Barrel Capacity

Rain falls vertically, so the water your roof collects depends on the flat footprint it covers rather than the sloped surface. One inch of rain over one square foot is 0.6233 gallons, which means a thousand square foot roof yields roughly 623 gallons per inch of rainfall before losses. This calculator returns the gross and net runoff for any storm you enter, the volume in cubic feet, the weight of that water in pounds, and the peak flow rate in gallons per minute using the rational method — coefficient times rainfall intensity times area in acres. Peak flow is the number that actually breaks drainage systems, because an outlet that cannot pass the rate causes the gutter to overshoot at the eave regardless of how large the gutter is. It also returns the minimum downspout count by capacity for both 2x3 and 3x4 outlets, how many rain barrels the storm would fill, and an annual rainwater harvest figure.