What This Solves
Sizes catch basins by calculating inlet capacity, outlet pipe capacity, and required sump storage for sediment and debris based on design flow and grate characteristics.
Best Used When
- You are designing a roadside or parking lot catch basin to intercept surface runoff
- You need to size the grate, sump depth, and outlet pipe for a point drainage inlet
- You want to verify that an existing catch basin has adequate capacity for a given design flow
Do NOT Use When
- You need to analyze a curb inlet or gutter interception along a roadway edge — Use Inlet Interception Calculator
- You are spacing multiple area drains in a yard or flat area — Use Area Drain Spacing Calculator
- You are designing a linear trench drain (channel drain with grate) rather than a point inlet — Use Trench Drain Calculator
Key Assumptions
- Grate capacity is based on orifice flow and weir flow equations for the specified grate open area
- Outlet pipe capacity is calculated using Manning's equation for gravity flow
- Sump storage volume is based on a user-specified sediment depth and cleanout frequency
- Inlet is fully effective (no bypass flow or partial clogging)
- Flow depth over the grate is small relative to the grate dimensions
Input Quality Notes
Actual grate capacity can be reduced by debris, ice, or partial clogging. Use a safety factor or select a larger grate size than calculated for critical locations.
Try a Common Scenario
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Catch Basin Calculator
Calculate catch basin sizing, outlet pipe capacity, and sediment storage requirements. Enter your design parameters to determine appropriate basin size, outlet pipe diameter, sump depth, and frame/grate recommendations.
Calculate Catch Basin Sizing
For educational purposes only. Not a substitute for professional engineering judgment.
Catch Basin Design Overview
Catch basins collect stormwater runoff and convey it to underground storm drain systems. This calculator sizes basins based on both inlet capacity and outlet capacity, identifying which is the limiting factor.
- Standard Catch Basin - Grate inlet on top, uses weir/orifice equations
- Curb Inlet - Opening in curb face, different hydraulic equations
- Combination - Both grate and curb opening for redundancy
- Area Drain - Smaller grate inlets for landscape/yard drainage
Note: Trench drains (linear features) require a different calculation approach and will be available in a separate calculator.
Standard Grate Sizes
| Size | Load Rating | Application |
|---|---|---|
| 6" x 6" | Light duty | Small landscape drains |
| 9" x 9" | Light duty | Residential yard drains |
| 12" x 12" | Class A/B | Residential/light commercial |
| 18" x 18" | Class B/C | Parking lots |
| 24" x 24" | Class C/D | Roadways |
| 30" x 30" | Class D/E | Heavy traffic areas |
Clogging Factor Guidelines
0% (Clean)
Theoretical maximum capacity
Use for capacity verification only
50% (Recommended)
Standard design assumption
Per FHWA HEC-22 guidance
75-90% (Severe)
Heavy debris conditions
Areas with leaf litter, construction
Traffic Load Ratings
Class A
2,500 lb capacity
Pedestrian areas only
Class B
10,000 lb capacity
Parking lots, driveways
Class C/D
40,000 lb capacity
Streets, highways
Class E/F
80,000 lb capacity
Airports, ports, heavy industrial
About Catch Basins
A catch basin (also called a storm drain inlet or drop inlet) is a drainage structure that collects surface runoff and directs it into the storm drainage system. Catch basins include a sump to trap sediment and debris before they enter the downstream piping.
Types of Catch Basins
- Standard Catch Basin - Grate inlet collecting surface flow
- Curb Inlet - Opening in the curb face for gutter flow
- Combination Inlet - Both grate and curb opening
- Area Drain - Yard or lawn drainage collection
- Trench Drain - Linear drainage for large areas
Key Design Equations
Manning's Equation (Outlet Pipe): Q = (k/n) * A * R2/3 * S1/2
Sump Volume: V = A * dsump
Where:
- Q = Flow rate (cfs or m3/s)
- k = Unit conversion factor (1.49 for US, 1.0 for SI)
- n = Manning's roughness coefficient
- A = Cross-sectional area of flow
- R = Hydraulic radius (A/P)
- S = Slope of pipe
- dsump = Sump depth below outlet invert
Design Considerations
- Outlet Capacity - Must accommodate design inflow rate
- Basin Size - Must fit outlet pipe plus working space
- Sump Depth - Minimum 12" recommended for sediment storage
- Frame/Grate - Load rating must match traffic conditions
- Inlet Capacity - Grate or curb opening must not limit flow
- Maintenance Access - Must allow for periodic cleaning
Resources
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Last verified: February 2026