When to use: Size a storm drain by computing the discharge a circular pipe carries when flowing just full. Using Manning's equation Q = (1.49/n)·A·R^⅔·S^½, with the hydraulic radius of a full circular pipe equal to D/4, gives the full-flow velocity and capacity. Storm pipes should also reach a minimum self-cleaning velocity of about 3 ft/s to prevent sediment deposition; the calculator flags whether the current slope achieves this and reports the minimum slope required.