Free calculator
Rational Method — Peak Runoff
Estimate the peak runoff rate from a small watershed. Enter the runoff coefficient, the rainfall intensity for a duration equal to the time of concentration, and the drainage area.
Qp = C · i · A
| C | runoff coefficient, 0–1 (area-weighted composite for mixed land use: C = ΣCjAj/ΣAj) |
| i | rainfall intensity for duration = time of concentration tc (from an IDF curve) |
| A | drainage area |
Best for small, fairly uniform watersheds (commonly under ~200 acres). The method assumes the whole watershed contributes — valid when the storm duration is at least tc.
For study and checking only. These calculators run entirely in your browser — nothing is sent anywhere. They are meant to verify your hand calculations while you study; on the exam you will work by hand with the NCEES reference handbook.