Storm Drain Sizing Calculator

This calculator combines two classic hydraulic tools to turn a drainage area into a pipe size. First, the Rational Method estimates peak design flow as Q = CiA — runoff coefficient C (the fraction of rain that becomes runoff, near 0.9 for pavement, as low as 0.2 for flat lawns) times rainfall intensity i (from local IDF curves for your chosen storm return period) times drainage area A in acres, conveniently yielding flow directly in cubic feet per second. That's a well-established but simplified approach valid for smaller watersheds (roughly under 200 acres) where travel time to the inlet is short and rainfall can be treated as spatially uniform.

Second, the calculator inverts Manning's equation for full-flow circular pipes — Q = (1.486/n)·A·R^(2/3)·S^(1/2), with pipe area A = πD²/4 and hydraulic radius R = D/4 for a full circle — algebraically solving for the diameter D that would carry exactly the design flow at your specified slope and Manning's roughness n (smooth concrete ≈0.012, corrugated metal ≈0.024, PVC/HDPE ≈0.010). Rather than spec a diameter to three decimal places, it then rounds up to the nearest standard commercial pipe size (8 through 96 inches) and recalculates that pipe's actual full-flow capacity and velocity — the number you'd check against a typical 2-10 ft/s self-cleaning, non-erosive velocity window. This assumes gravity flow at full pipe capacity; it doesn't check partial-flow depth, surcharge/inlet conditions, or minimum cover, all of which a complete storm drain design still needs to verify.