Flowpath Engineering Portal
Rational Formula Runoff Calculator — Peak Flow Estimation
Estimate peak stormwater runoff using the Rational Method (Q = CiA). Calculate design flows with NZ rainfall intensities, runoff coefficients, and climate change adjustments.
What this tool does
Rational Method (Q = CiA)
Calculate peak runoff using the classic Rational Formula with catchment area, runoff coefficient, and rainfall intensity inputs.
NZ Rainfall Integration
Use HIRDS rainfall intensity data for any New Zealand location, with automatic lookup by ARI and storm duration.
Climate Change Adjustments
Apply RCP-based climate change factors to rainfall intensities for future-proofed stormwater design in accordance with NZ guidelines.
Multiple Sub-Catchments
Combine flows from multiple sub-catchments with different land uses, slopes, and runoff characteristics into a single design flow.
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Open Rational Formula CalculatorPart of the Flowpath Engineering suite of free civil engineering tools for New Zealand.