Professional Drainage Sizing — Fast and Accurate
Quick field sizing tools backed by NRCS, FHWA, and ASCE standards. Get accurate pipe sizes, drain capacities, and pond volumes you can use in bids and proposals.
Most-Used Contractor Tools
The calculators contractors reach for most often. All include shareable URLs and unit toggling.
Rational Method Calculator
Enter runoff coefficient, rainfall intensity, drainage area, and return period to calculate Rational Method peak discharge (Q = CiA), adjusted C, and a transparent calculation breakdown.
Open CalculatorManning's Equation for Pipe Flow Calculator
Enter circular pipe diameter, slope, roughness, and flow depth to calculate full- or partial-flow discharge, velocity, and hydraulic properties with Manning's equation.
Open CalculatorFrench Drain Calculator
Enter trench, aggregate, pipe, and soil parameters to calculate French drain storage, infiltration, and pipe capacity using Darcy's law and Manning's equation.
Open CalculatorCatch Basin Calculator
Enter design inflow and inlet geometry to estimate HEC-22 weir/orifice inlet capacity, then screen outlet-pipe capacity with Manning's equation and preliminary basin and sump dimensions.
Open CalculatorTrench Drain Calculator
Calculate trench drain capacity using Manning's equation.
Open CalculatorVegetated Swale Calculator
Calculate vegetated swale hydraulics including normal depth, velocity, and capacity using Manning's equation with variable roughness based on vegetation retardance.
Open CalculatorPond Sizing Calculator
Enter watershed and release criteria to estimate preliminary detention storage with the Modified Rational Method and size orifice and weir outlets.
Open CalculatorDownspout Sizing Calculator
Enter roof area and rainfall intensity to select round or rectangular downspout sizes from IPC sizing tables.
Open CalculatorManning's Channel Calculator
Enter channel geometry, slope, roughness, and depth to calculate discharge and velocity with Manning's equation for rectangular, trapezoidal, triangular, or parabolic sections.
Open CalculatorInlet Interception Calculator
Enter gutter flow and grate, curb-opening, or combination-inlet geometry to calculate HEC-22 interception efficiency, captured flow, and bypass flow.
Open CalculatorBuilt for the Field
Shareable URLs
Every calculation generates a URL you can share with clients or include in proposals.
Unit Toggle
Switch between imperial and metric units with one click. Results update instantly.
Standard Methods
Every calculator uses published methods from NRCS, FHWA, ASCE, and other authorities.
Reference Tables
Quick access to Manning's n, pipe sizes, runoff coefficients, and soil properties.
Quick Reference Tables
The values you look up most often, all in one place.
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