stormwatercenter.net · printed Jun 12, 2026
Simple Method Runoff Calculator
What the Simple Method does
The Simple Method estimates annual stormwater runoff volume and annual pollutant loads from a small urban drainage area using only inputs a planner actually has on hand: drainage area, impervious cover, annual rainfall and a pollutant concentration. It deliberately trades precision for transparency — every number in the chain can be checked by hand, which is why it remains the standard screening tool for subwatershed planning, TMDL scoping and before/after comparisons of development scenarios.
The equations
Annual runoff is the product of rainfall, the fraction of events large enough to produce runoff, and a runoff coefficient driven by impervious cover:
The 0.226 constant converts inch-acres of runoff times milligrams per liter into pounds. For bacteria the load equation uses its own unit constant, which the calculator applies automatically.
Where the concentrations come from
The default event mean concentrations (EMCs) for residential, commercial, industrial and roadway land uses are national defaults compiled from nationwide monitoring data, including the Nationwide Urban Runoff Program. They are intended as starting values: replace them with local or regional monitoring data whenever it exists — the calculator’s user-defined mode accepts any concentration set. For treatment performance once a practice is in place, see the pollutant removal database.
Limits worth respecting
- Scale: the method is calibrated for drainage areas up to about one square mile. Beyond that, channel processes and baseflow start to dominate and a watershed model is the right tool — see the Watershed Treatment Model.
- Annual averages only: the output is a long-term annual load, not a design storm. For sizing practices use the water quality volume framework in the sizing criteria guide.
- Impervious cover drives everything: the method’s sensitivity to I is the point — it makes the connection between land cover and load explicit. The Impervious Cover Model explains what those percentages mean for stream health.
When comparing development scenarios, hold P, Pj and C constant and let only impervious cover vary — the relative difference between scenarios is far more robust than any single absolute load estimate.
Typical applications
- Ranking subwatersheds by relative pollutant load to prioritize retrofits
- Estimating load reductions from better site design measures that lower impervious cover
- Quick checks of consultant model output — if a sophisticated model disagrees with the Simple Method by an order of magnitude, something is usually wrong with the inputs
- Public communication: the chain from rooftops to pounds of phosphorus fits on one slide
Frequently asked questions
What is Pj and why is it 0.9?
Pj is the fraction of annual rainfall events large enough to produce measurable runoff. Roughly 10% of events are too small to generate runoff from typical urban surfaces, so 0.9 is the standard default.
Can the Simple Method be used for a single storm?
No — it produces annual averages. For event-based work use curve-number hydrology, as in the worked sand filter example.
What counts as impervious cover?
Rooftops, roads, driveways, parking and other surfaces that prevent infiltration. Measured imperviousness from aerial imagery beats land-use lookup tables when available.
How accurate are the loads?
Within a factor of two is a realistic expectation with default concentrations. Local EMC data tightens that considerably. The method’s strength is comparative analysis, not absolute prediction.
Does the method account for stormwater treatment?
Not directly — it estimates the load generated. Apply removal efficiencies from the pollutant removal database to estimate the load after treatment.
Why do roadway concentrations differ from residential?
Vehicle wear, fuels and de-icing produce systematically higher sediment and metal concentrations on roadways. The land-use presets reflect those monitored differences.