Rational Method – Multi‑Subcatchment Peak Flow

Area‑weighted Rational method calculator for small urban and mixed catchments. Combine several subcatchments with different runoff coefficients into a single design peak discharge.
Units: SI (ha, mm/hr, m³/s) or US (acres, in/hr, cfs).

1. Setup & Design Intensity Units & Storm

Select unit system and enter design rainfall intensity. Intensity should correspond to the critical storm duration (≈ time of concentration) and chosen return period from your IDF curves.
Match this with the units of your IDF and catchment plan.
In mm/hr (SI) or in/hr (US), consistent with your time of concentration Tc and chosen AEP.
For documentation only – Rational method assumes i corresponds to Tc.
Used in XLSX / PDF export.

2. Subcatchments Areas & Runoff Coefficients

Enter up to six subcatchments. The first row is required. Rows 2–6 are optional; if you fill either area or C, both must be valid. Runoff coefficients C should normally be between 0 and 1.
# Name / land‑use Area A
(ha)		 Runoff coeff. C
(0–1)		 Notes / soil group
(optional)
1
2
3
4
5
6

Example: roofs C ≈ 0.9, pavements ≈ 0.8–0.95, lawns ≈ 0.2–0.4 depending on slope and soil group. Always check your local stormwater design guide for recommended C values.