Rational Method Runoff

No -- in this calculator's metric units (Q in m^3/s, rainfall intensity in mm/hr, area in hectares), 360 is the exact algebraic conversion factor, not a rounded constant. The commonly cited imperial version, Q = CiA with Q in cfs, i in in/hr, and A in acres, actually carries a small rounding (1 acre-inch/hour is about 1.008 cfs, conventionally dropped to 1); the metric form used here has no equivalent rounding to drop.

It is Peak Discharge multiplied by 3,600 seconds -- the volume you would get if the peak flow rate held perfectly constant for one full hour. Real storms don't produce a rectangular hydrograph; discharge rises to a peak and falls off on both sides, so the true volume of runoff from an actual storm is generally less than this figure. Treat it as an upper-bound planning number, not a metered volume.

Treat them as illustrative starting points from general civil engineering references, not an approved regulatory table. Runoff coefficients that matter for a real drainage permit come from your local jurisdiction's drainage design manual, which sets C by soil group, slope, and land-use category specific to that city, county, or state DOT -- those values can differ meaningfully from generic textbook examples, so confirm against the local manual before submitting anything.

Once the catchment gets large, has mixed land cover, or has flow paths long enough that the storm duration can no longer be assumed to match the time of concentration across the whole area, the Rational Method's core assumption breaks down. Most agencies cap its use at catchments on the order of a few hundred hectares; beyond that, or when you need a full runoff hydrograph rather than one peak number, NRCS curve-number or hydrograph-based methods are the standard replacement.