The runoff coefficient C answers one question per surface: of the rain that falls, how much runs off instead of soaking in? Hard surfaces score high — asphalt and concrete pavement 0.85–0.95, roofs 0.75–0.95 — while pervious covers drop fast: gravel 0.35–0.70, dense residential 0.50–0.70, lawns on steep heavy soil 0.25–0.35, and flat sandy lawns as low as 0.05–0.10.
C is the land-use dial in the rational method, the small-catchment peak-flow standard: Q = C·i·A in US customary units (cfs, in/hr, acres — the classic identity that 1 acre at 1 in/hr gives about 1 cfs), or Q = 0.278·C·i·A in SI with intensity in mm/hr and area in km². A mostly paved 1 ha commercial site at C = 0.9 under a 100 mm/hr design storm produces 0.278 × 0.9 × 100 × 0.01 ≈ 250 L/s of peak flow for the storm drain to carry.
Real sites mix covers, so practice builds an area-weighted composite: each sub-area’s C times its area, summed and divided by the total. The coefficient even sneaks into timing — the FAA time-of-concentration formula tc = 1.8·(1.1 − C)·L^0.5/S%^(1/3) uses it, so a more impervious surface concentrates flow faster as well as shedding more of it.