Runoff Calculator
Quick storm checks. Choose Metric or US units.
Units
Metric (SI)
US (Imperial)
Volume (Storm)
Peak Flow (Rational)
Rainfall depth (P)
Event rainfall depth
Catchment area (A)
Plan area draining to your point
Runoff coefficient (C: 0–1)
0 = full soak, 1 = no soak
Storage capacity (optional)
Used for quick OK / Not OK check
Runoff Volume
Alt. Volume
Depth × Area × C
Formula: V = (P/1000) · A · C. P in mm, A in m² → V in m³.
Rain intensity (i)
Design intensity for the storm
Catchment area (A)
See unit by system
Runoff coefficient (C: 0–1)
Pipe/Channel capacity (optional)
For OK / Not OK check
Peak Flow
Alt. Flow
Rational: C · i · A
Rational: Q (m³/s) = C · i(mm/h) · A(m²) / 3,600,000.