Calculate a storm volume
Roof catchment: when using this for a roof, enter the horizontal projected/plan area that receives rainfall—not the sloped surface area of the roofing.
What the coefficient means
The coefficient is the fraction of rainfall treated as direct runoff in this simplified volume model. A roof is commonly screened with a high value, while lawn response varies much more with soil, compaction, slope, vegetation and antecedent moisture. Wrivel makes the coefficient editable because a single “correct” lawn value does not exist for every property.
The conversion
The factor comes directly from volume conversion: one inch over one square foot equals 1/12 cubic foot, then cubic feet are converted to US gallons.
What this number can and cannot do
Use the volume to compare storms, estimate a downspout catchment or set a first-pass storage target. It does not calculate peak flow rate, pipe capacity, inlet capacity, infiltration rate or a permit-ready stormwater design.
Quick examples that make the scale visible
Use contributing area, not the whole property by default
A downspout branch may receive only one roof section. A driveway drain may receive only the pavement uphill of that inlet. Enter the area that actually contributes to the problem you are sizing, not every square foot you own.
Why volume is useful before product selection
Volume tells you the scale of the water problem. It can show whether a small barrel will fill almost immediately, whether a storage concept is orders of magnitude too small, or whether redirecting one roof branch would materially reduce water reaching a wet area.
Volume alone does not tell you how fast water arrives. A short intense storm and a long gentle storm can have the same total depth but very different peak flow rates, which matters for gutters, inlets and pipes.