Runoff Coefficients (C) for the Rational Method
The rational method, Q = CiA, estimates peak stormwater flow from a small site, and C is the fraction of rainfall that runs off. Pavement and roofs shed most of the rain, while lawns absorb most of it. These are the classic ranges published by ASCE and the Water Pollution Control Federation, which many local drainage manuals still reproduce.
| Surface or land use | C, low | C, high |
|---|---|---|
| Business, downtown areas | 0.7 | 0.95 |
| Business, neighborhood areas | 0.5 | 0.7 |
| Residential, single-family areas | 0.3 | 0.5 |
| Residential, multi-units detached | 0.4 | 0.6 |
| Residential, multi-units attached | 0.6 | 0.75 |
| Residential, suburban | 0.25 | 0.4 |
| Apartment dwelling areas | 0.5 | 0.7 |
| Industrial, light | 0.5 | 0.8 |
| Industrial, heavy | 0.6 | 0.9 |
| Parks, cemeteries | 0.1 | 0.25 |
| Playgrounds | 0.2 | 0.4 |
| Railroad yards | 0.2 | 0.4 |
| Unimproved areas | 0.1 | 0.3 |
| Lawns, sandy soil, flat (under 2%) | 0.05 | 0.1 |
| Lawns, sandy soil, average (2% to 7%) | 0.1 | 0.15 |
| Lawns, sandy soil, steep (over 7%) | 0.15 | 0.2 |
| Lawns, heavy soil, flat (under 2%) | 0.13 | 0.17 |
| Lawns, heavy soil, average (2% to 7%) | 0.18 | 0.22 |
| Lawns, heavy soil, steep (over 7%) | 0.25 | 0.35 |
| Streets, asphalt | 0.7 | 0.95 |
| Streets, concrete | 0.8 | 0.95 |
| Streets, brick | 0.7 | 0.85 |
| Drives and walks | 0.75 | 0.85 |
| Roofs | 0.75 | 0.95 |
Notes
- Values apply to storms of about 10-year frequency or less. For less frequent storms many manuals multiply C by a frequency factor: 1.1 for 25-year, 1.2 for 50-year and 1.25 for 100-year, with C x Cf not to exceed 1.0.
- For a site with mixed surfaces, compute an area-weighted composite C.
- Your city or county drainage manual governs. Many jurisdictions publish their own C values, and those must be used for permit submittals.
- In Q = CiA with i in in/hr and A in acres, Q is in cfs (the exact factor is 1.008, usually taken as 1.0).
Frequently asked questions
What runoff coefficient should I use for a roof?
The published range is 0.75 to 0.95. Many local manuals specify 0.90 or 0.95 for roofs and pavement.
Why does adding pavement require a detention pond?
Replacing lawn (C around 0.1 to 0.3) with pavement (0.7 to 0.95) can triple or more the peak runoff. Most jurisdictions require the developed peak not to exceed the pre-development peak, so the difference has to be detained.
Is the rational method allowed for large sites?
It is meant for small drainage areas. Many agencies cap it at about 20 to 200 acres; check the local manual.
Sources
- ASCE and WPCF, Design and Construction of Sanitary and Storm Sewers, ASCE Manual of Practice No. 37 (1969), as reproduced in ASCE/WEF Manual of Practice No. 77 (1992)
- PDHengineer course C-2040, Estimating Stormwater Runoff, Table 3a and 3b
More reference tables
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