Storm drain calculator — peak runoff rate
Enter your drainage area, surface type, and rainfall intensity. The calculator uses the Rational Method to give you peak flow in cfs, gpm, or litres per second — instantly.
Using the Rational Method: C = 0.8, i = 50 mm/hr, A = 200 m².
Q = C × i × A ÷ 3,600,000 (SI). Rational Method is appropriate for small catchments up to ~200 acres (80 ha). For large or complex sites, consult a licensed civil engineer.
How the calculation works
The Rational Method is the industry-standard formula for small catchments:
Q = C × i × A
- Q = peak runoff rate (cfs or L/s)
- C = runoff coefficient (dimensionless, 0–1)
- i = rainfall intensity (in/hr or mm/hr)
- A = drainage area (acres or hectares)
The runoff coefficient (C) represents what fraction of rain becomes runoff. Hard surfaces like asphalt (C = 0.80) shed most water; wooded areas (C = 0.15) absorb most of it. Rainfall intensity comes from local IDF (intensity-duration-frequency) curves for your design storm — 2-year, 10-year, or 100-year return period depending on the application.
Want the full walk-through? Read the stormwater runoff guide →
Reference & education only. Not professional, engineering, or code-compliance advice. Estimates are based on published model codes; local amendments and your Authority Having Jurisdiction (AHJ) govern. Always verify against the current adopted code and a licensed professional before doing work.
Last reviewed 2026-08.