When to use: The Rational Method estimates peak storm runoff for small drainage areas (generally < 200 acres). Q = C·i·A, where C is the composite runoff coefficient, i is rainfall intensity (in/hr) from the IDF curve at the time of concentration, and A is the drainage area in acres. With these units Q comes out directly in cubic feet per second (the 1.008 conversion factor is taken as 1.0). Use a composite C weighted by sub-area for mixed land cover.