Drainage Flow Calculator
Not All Ground Sheds Rain the Same Way
The Rational Method estimates peak stormwater runoff from a simple premise: a surface's runoff coefficient describes what fraction of falling rain actually becomes runoff rather than soaking in. Asphalt and roofing sheds around 90% of what falls on them; a flat, sandy lawn absorbs most of it and sheds closer to 10%. Multiplying that coefficient by rainfall intensity and drainage area gives a standard, widely used estimate of peak flow for sizing storm drains and culverts on small watersheds.
The Formula
(C = runoff coefficient, i = rainfall intensity in in/hr, A = area in acres, Q in cfs)
Runoff Coefficients by Surface Type
| Surface | Runoff coefficient (C) |
|---|---|
| Pavement / Asphalt | 0.90 |
| Roof | 0.90 |
| Gravel | 0.50 |
| Cultivated land | 0.30 |
| Lawn, heavy soil, steep (>7%) | 0.35 |
| Lawn, heavy soil, average (2–7%) | 0.22 |
| Lawn, heavy soil, flat (<2%) | 0.17 |
| Woodland | 0.15 |
| Lawn, sandy soil, average (2–7%) | 0.15 |
| Lawn, sandy soil, steep (>7%) | 0.20 |
| Lawn, sandy soil, flat (<2%) | 0.10 |
Pavement and roofing produce roughly nine times the runoff of a flat, sandy lawn for the same rainfall — which is why converting undeveloped land to pavement is one of the most direct ways a site increases downstream drainage demand.
Where This Gets Used
- Sizing storm drains and culverts — the Rational Method is a standard first step for small drainage areas, typically under roughly 200 acres.
- Comparing pre- and post-development runoff — estimating how much peak flow increases when undeveloped land is converted to pavement or rooftop.
- Mixed-surface site estimates — running the calculation separately for each surface type on a site and summing the results for a composite peak flow.
How to Use This Calculator
- Select Surface Type from the list, or enter a Custom Runoff Coefficient to override it.
- Enter Rainfall Intensity in inches per hour.
- Enter Drainage Area in acres.
- Select Calculate to see the peak flow.
Related Calculations
Size a culvert for this flow with the Culvert Sizing Calculator, or check pavement thickness for a paved surface driving this runoff with the Pavement Thickness Calculator.