🌧️ Stormwater Runoff

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 1 independent reference calculation · how PhDino checks its numbers

Related standards: Rational Method

Estimating peak runoff from a small drainage area using the Rational Method.

When rain falls on a piece of land, some of it soaks in and some of it runs off across the surface. The Rational Method is the oldest and still most widely used way to estimate that peak runoff rate for small drainage areas — small enough (typically under about 200 acres) that the whole area can be treated as responding to rainfall all at once, rather than needing a full hydrograph.

The runoff coefficient C captures how much of the rainfall actually becomes runoff rather than soaking in — pavement and rooftops have a C close to 1 (almost everything runs off), while a lawn or wooded area has a much lower C (much of it infiltrates). Rainfall intensity i comes from local intensity-duration-frequency data for whatever storm return period the project needs to be designed for (a 10-year storm, a 100-year storm, and so on).

One tidy fact about the Rational Method: when C is dimensionless, i is in inches per hour, and A is in acres, the product C·i·A comes out in cubic feet per second without any conversion constant — 1 acre-inch per hour happens to equal almost exactly 1 cubic foot per second (the two units are close enough, within about half a percent, that engineers use the formula directly). That numerical coincidence, not an approximation, is why the formula looks so clean in US customary units.

Key formula

Q = C × i × A

Variables

C

runoff coefficient — fraction of rainfall that becomes runoff, based on land cover

i

design rainfall intensity for the chosen storm return period

A

drainage area contributing runoff to the point being checked

Notes & limitations

• The Rational Method assumes the whole drainage area contributes runoff simultaneously, which breaks down for large or oddly-shaped watersheds — those need a more detailed hydrologic model instead.
• A single C value stands in for a whole drainage area, which is a simplification when the area mixes very different surfaces (roof, driveway, lawn) — a weighted-average C by sub-area gives a better estimate than eyeballing one overall value.