Storm Drain Pipe Sizer

Need to size a storm drain pipe but tired of flipping through design manuals? This calculator does the hydraulics for you in seconds. Enter rainfall intensity, drainage area, runoff coefficient, pipe slope, and material to get the exact pipe diameter your project needs. No more guessing between standard sizes. WHAT YOU GET: - Peak runoff flow (Q) via the Rational Method - Required pipe diameter from Manning equation - Nearest standard pipe size recommendation - Full-pipe flow velocity and capacity check - Self-cleansing velocity verification SURFACE TYPES: Rooftop, concrete, gravel, flat lawn, steep lawn, and forest — each with standard runoff coefficients built in. PIPE MATERIALS: Concrete, PVC, corrugated metal, and HDPE with accurate Manning roughness values. STANDARD SIZES: Recommends from 4-inch through 60-inch standard pipe diameters. Works completely offline. No account required. Built for civil engineers, stormwater designers, and site contractors who need fast, reliable pipe sizing on the job.