Stormwater Runoff Calculator

Calculate stormwater runoff using the Rational Method (Q=CIA) for drainage design — free hydraulic calculation tool

Runoff Coefficient Presets

Rational Formula Parameters

Runoff Coefficient C

Rainfall Intensity I (mm/hr)

Catchment Area A (ha)

Time of Concentration

Flow Path Length L (m)

Slope S (%)

Time of Concentration tc

23.6

min

tc = L / (60·√(S/100))

Calculation Results

Design Runoff Q

0.1181

m³/s

(118.1 L/s)

Suggested Pipe Dia.

400

(at V = 1.0 m/s)

Rational Formula:

Q = (1/360) · C · I · A

Q = (1/360) · 0.85 · 50 · 1

Q = 0.1181 m³/s

Where Q (m³/s), C (dimensionless), I (mm/hr), A (ha)

Runoff Coefficient Reference