Rational Method – Peak Discharge Tool

Quick calculator for design peak runoff from small catchments using the Rational Method.
Supports SI and US customary forms (Q = 0.278 C I A or Q = 1.008 C I A).

1. Inputs Catchment & Rainfall

Choose unit system, then provide catchment area, runoff coefficient, and design rainfall intensity.
Calculations always show both m³/s and cfs, regardless of selected input system.
Small catchments Urban drainage Culverts & bridges
Typical range 0.1–0.9 depending on land use and imperviousness.
Use intensity for storm duration ≈ time of concentration, from IDF curves.
Area draining to the design outlet in km².
Used only for reporting. Intensity i should correspond to this tc.
For documentation of design conditions (e.g. 10, 25, 50, 100 years).
Design discharge = k × Q. If left blank, k = 1.0.