Drainage CalculatorsPrecision Planning Tools
Professional hydraulic calculators designed to help engineers and contractors plan effective drainage solutions with precision and confidence.
Instant Results
Real-time calculations update as you adjust parameters.
Location-Aware
ZIP code lookup for local rainfall intensity data from NOAA.
Engineer Approved
Based on industry-standard methods and equations.
Choose Your Calculator
Modified Rational Method
Runoff Calculator
Calculate peak flow rates and total runoff volume based on local rainfall data, drainage area, and surface conditions. Essential for sizing catch basins and determining pipe capacities.
Key Features
- Automatic rainfall intensity lookup by ZIP code
- Multiple return period analysis (2-year to 100-year)
- Common surface type coefficients
- Recommended pipe sizing based on flow rates
Common Applications
- Sizing catch basins and area drains
- Determining required pipe capacities
- Stormwater detention calculations
- Municipal permit requirements
Channel & Outlet Analysis
Trench Drain Sizing
Comprehensive analysis tool for sizing trench drains, considering both channel capacity and outlet configurations. Combines runoff calculations with hydraulic analysis.
Key Features
- Combined runoff and hydraulic calculations
- Channel and outlet capacity analysis
- Standard trench sizes and materials
- Real-time adequacy assessment
Common Applications
- Driveway and parking lot drainage
- Industrial facility design
- Sports field drainage systems
- Linear drainage optimization
Manning's Equation Analysis
Flow Calculator
Analyze open channel hydraulics using Manning's equation and flow regime characteristics. Calculate flow rates, velocities, and determine subcritical or supercritical conditions.
Key Features
- Manning's equation calculations
- Flow regime analysis (Froude number)
- Velocity and depth relationships
- Multiple channel materials
Common Applications
- Channel design verification
- Erosion control assessment
- Sediment transport analysis
- Hydraulic jump prediction
Manning's Equation for Circular Pipe
Pipe Sizing Calculator
Size storm drain pipe with Manning's equation for full and part-full circular sections. Compute capacity (CFS/GPM) and velocity, and find the smallest standard diameter that carries your design flow.
Key Features
- Full-pipe capacity via Q = (1.49/n)·A·R^(2/3)·S^(1/2)
- Exact part-full circular geometry (peak Q near 94% depth)
- Required-diameter search across standard sizes
- Self-cleaning vs. erosion velocity checks
Common Applications
- Storm drain trunk and lateral sizing
- Culvert capacity verification
- Outfall pipe selection
- Velocity / scour screening
Modified Rational Method
Detention Volume Calculator
Size required underground detention storage from post-development inflow and your permitted release rate. Results in cubic feet, gallons, and acre-feet for matching chamber capacities.
Key Features
- Modified Rational Method: V = (CIA − Q_allow)·t_d
- Inflow vs. released volume breakdown
- Output in ft³, gallons, and acre-feet
- Critical-duration screening guidance
Common Applications
- Underground detention chamber sizing
- Post-construction stormwater compliance
- Release-rate (pre vs. post) matching
- Permit volume documentation
Outlet Control Discharge
Weir & Orifice Calculator
Compute discharge through outlet control structures: rectangular and V-notch weirs and sharp-edged orifices. Essential for designing detention outlets that meet allowable release rates.
Key Features
- Rectangular weir Q = Cw·L·H^(3/2)
- V-notch weir Q = Cw·tan(θ/2)·H^(5/2)
- Orifice Q = Cd·A·√(2gH)
- Results in CFS and GPM with input validation
Common Applications
- Detention outlet structure design
- Multi-stage outlet control
- Allowable release-rate sizing
- Flow measurement structures
Which Calculator Should I Use?
Our planning tools are designed to work together for comprehensive drainage solutions. Here's a recommended workflow:
Start with the Runoff Calculator
Determine flow rates based on your drainage area and local rainfall data.
Size Your Trench Drain
Use the Trench Drain Sizing Calculator to select appropriate channel dimensions and outlets.
Verify Flow Characteristics
Use the Flow Calculator to analyze hydraulic properties and ensure proper flow regime.
Need Help Choosing?
Our drainage specialists can help you interpret calculations and select the right products for your specific application.