Free calculator
Rational Method Peak Flow Calculator
The standard small-catchment peak flow estimate used across South African stormwater and drainage design — with the unit conversions handled for you.
Rational Method Peak Flow Calculator
About this tool: Computes the Rational Method peak discharge Q = C·i·A / 3.6 for small catchments, per the SANRAL Drainage Manual convention (Q in m³/s, i in mm/h, A in km² — hectares are converted for you).
Peak discharge Q
= 0.55 × 95 mm/h × 0.45 km² / 3.6
The formula
Q = C · i · A / 3.6 — Q the peak discharge (m³/s), C the runoff coefficient (dimensionless), i the average rainfall intensity (mm/h) for the critical storm duration, A the catchment area (km²). The 3.6 is purely a unit conversion; with A in hectares the divisor is 360.
Worked example
A 45 ha (0.45 km²) suburban catchment, composite C = 0.55, 1:50-year intensity of 95 mm/h at the catchment's time of concentration: Q = 0.55 × 95 × 0.45 / 3.6 ≈ 6.5 m³/s.
The method suits small catchments (indicatively up to ~15 km²) and peak-only questions. For how to build C, source i, and when to switch methods, see our Rational Method explainer — and estimate the critical duration with the time of concentration calculator.
Disclaimer
This tool produces screening-level estimates from standard published formulas. It is not a design deliverable, does not account for site-specific conditions, and must not be used as the basis for design, construction, or regulatory submissions without professional review. Delta Hydro Engineers (Pty) Ltd accepts no liability for decisions taken on unverified calculator output.
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