Catch 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.

Calculate Catch Basin Sizing

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Basin Type

Select the type of inlet structure

Traffic Loading

Expected traffic for frame selection

Grate Configuration

Grate Size

Select standard size or specify custom

Clogging Factor

0 = clean, 0.5 = 50% clogged (recommended), 0.9 = heavily clogged

Assumed % of grate clogged (50% typical for design)

Flow Requirements

Design Inflow

cfs

Peak inflow rate the basin must handle

Outlet Slope

Slope of outlet pipe

Ponding Depth Calculation

Ponding depth affects inlet capacity. Specify directly OR provide drainage area and rainfall to calculate.

Ponding Depth

Direct input (or leave blank to calculate)

Drainage Area

acres

Contributing area (for ponding calculation)

Rainfall Intensity

in/hr

Design storm intensity

Runoff Coefficient

C value for rational method

Pipe Configuration (Optional)

Specify outlet pipe size if known, or let the calculator size it automatically.

Number of Inlet Pipes

Pipes entering the catch basin

Inlet Pipe Diameter

Diameter of inlet pipe(s)

Outlet Pipe Diameter

Diameter of outlet pipe (or auto-size)

Additional Options

Required Sediment Storage

Minimum sump volume

Minimum Sump Depth

Override default sump depth

Frame Material

Material for grate/cover frame

Structure Material

Material for catch basin body

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