National Pollutant Discharge Elimination System (NPDES)

National Pollutant Discharge Elimination System (NPDES)

Rainfall Erosivity Factor Calculator for Small Construction Sites

EPA’s stormwater regulations allow NPDES permitting authorities to waive NPDES permitting requirements for stormwater discharges from small construction sites if:

  • the construction site disturbs less than five acres, and
  • the rainfall erosivity factor (“R” in the revised universal soil loss equation, or RUSLE) value is less than five during the period of construction activity.

If your small construction project is located in an area where EPA is the permitting authority and your R factor is less than five, you qualify for a low erosivity waiver (LEW) from NPDES stormwater permitting. If your small construction project does not qualify for a waiver, then NPDES stormwater permit coverage is required. Follow the steps below to calculate your R-Factor.

LEW certifications are submitted through the NPDES eReporting Tool or “CGP-NeT”. Several states that are authorized to implement the NPDES permitting program also accept LEWs. Check with your state NPDES permitting authority for more information.

The R-factor calculation can also be integrated directly into custom applications using the R-Factor web service.

For questions or comments, email EPA’s CGP staff at

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Facility Information

Date Lat/Long
Start Date: Latitude:
End Date: Longitude:

Calculation Results

Rainfall erosivity factor (R Factor) =