Runoff Coefficient Calculator
Calculate the "Weighted C" value for your SWPPP hydrology analysis. Based on EPA Appendix C methodology.
Site Surface Areas
Enter the acreage for each surface type on your site.
Weighted 'C' Value
Enter your site data to calculate.
Why this matters
The EPA requires estimating runoff before and after construction. A higher 'C' means more runoff volume, which requires larger sediment basins.
Automate your SWPPP →Understanding the Runoff Coefficient
The Runoff Coefficient (C) is a dimensionless number between 0 and 1 that represents the percentage of rainfall that becomes runoff versus the amount that infiltrates the ground.
- C = 0.95: Almost all rain runs off (e.g., Concrete, Asphalt).
- C = 0.15: Most rain soaks in (e.g., Grass, Forest).
Why calculate a "Weighted" C?
Construction sites are rarely uniform. You might have 2 acres of paved parking, 1 acre of building roof, and 5 acres of bare soil. To properly size your Sediment Basins and drainage channels, you must calculate a single 'C' value that represents the average of the entire site area.
The Rational Method Formula
Engineers use the Rational Method to estimate peak discharge (Q):
Q = C × i × A
- Q: Peak discharge (cubic feet per second)
- C: Weighted Runoff Coefficient (from this calculator)
- i: Rainfall intensity (inches per hour)
- A: Drainage area (acres)
If your 'C' value is too low, you will undersize your basins, leading to overflows and EPA fines during storm events.