Catch Basin Size Calculator
Size storm water catch basins from drainage area, runoff coefficient, and rainfall intensity. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
Q = C x I x A (Rational Method)
Where Q is peak discharge in cubic feet per second (cfs), C is the dimensionless runoff coefficient (0 to 1), I is rainfall intensity in inches per hour for the design storm, and A is the drainage area in acres. The catch basin is then sized to intercept this peak flow rate.
Worked Examples
Example 1: Commercial Parking Lot Drainage
Problem:A 2.5-acre commercial parking lot (C=0.85) in a region with 10-year storm intensity of 4.5 in/hr. Size the catch basins needed.
Solution:Peak Flow Q = C x I x A\nQ = 0.85 x 4.5 x 2.5 = 9.56 cfs\n\nFor Q = 9.56 cfs, recommended basin size: 30 x 30 inches\nGrate area = (30 x 30) / 144 = 6.25 sq ft\nRecommended outlet pipe: 18-inch diameter\nSump depth: 24 inches minimum
Result:30 x 30 inch basin | 18-inch outlet pipe | Peak flow: 9.56 cfs
Example 2: Residential Street Inlet Design
Problem:A residential street drains 1.2 acres (C=0.55) with rainfall intensity of 3.5 in/hr and 2% longitudinal slope.
Solution:Peak Flow Q = C x I x A\nQ = 0.55 x 3.5 x 1.2 = 2.31 cfs\n\nFor Q = 2.31 cfs, recommended basin size: 24 x 24 inches\nGrate area = (24 x 24) / 144 = 4.0 sq ft\nRecommended outlet pipe: 12-inch diameter\nSump depth: 18 inches minimum
Result:24 x 24 inch basin | 12-inch outlet pipe | Peak flow: 2.31 cfs
Frequently Asked Questions
What is a catch basin and what purpose does it serve in stormwater management?
A catch basin is a drainage structure installed at low points along streets, parking lots, and other paved areas to collect surface runoff and direct it into the underground storm sewer system. It consists of a concrete or precast box with a grated inlet on top, a sump at the bottom to trap sediment and debris, and an outlet pipe connection to the storm drain network. Catch basins prevent flooding by intercepting stormwater before it accumulates on road surfaces, and the sump helps protect downstream pipes from clogging by settling out sand, gravel, leaves, and other debris. They are critical infrastructure elements in urban drainage systems.
What factors determine the required size of a catch basin?
The required catch basin size depends on the peak stormwater flow rate, the grate inlet capacity, the outlet pipe size, and local regulations. The peak flow rate determines the minimum grate opening area needed to intercept the design flow without excessive bypass. Standard catch basin sizes range from 18 by 18 inches for low-flow residential applications to 48 by 48 inches or larger for high-flow commercial and road drainage situations. The outlet pipe diameter must be large enough to convey the peak flow at acceptable velocities, typically between 2.5 and 10 feet per second. The basin depth must accommodate the sump for sediment storage, the outlet pipe, and sufficient freeboard above the pipe to prevent surcharging.
How often should catch basins be cleaned and maintained?
Catch basins should be inspected and cleaned at least twice per year, with additional cleanings after major storm events or in areas with heavy sediment loading. Most municipalities recommend cleaning when the sump is 50 percent full of sediment and debris, as a full sump eliminates the settling function and allows pollutants to pass through to downstream waterways. The cleaning process involves removing accumulated sediment, debris, and standing water using a vacuum truck (vactor), then inspecting the basin structure for cracks, joint separation, and pipe connections. Neglected catch basins can cause street flooding, pipe blockages, and water quality violations. Many communities now use GPS-tracked maintenance programs to optimize cleaning schedules.
What is the difference between a catch basin and a storm drain inlet?
While the terms are sometimes used interchangeably, there is a technical distinction between catch basins and storm drain inlets. A catch basin has a sump (a recessed bottom section below the outlet pipe) that traps sediment, debris, and some pollutants before they enter the storm sewer system. A storm drain inlet, on the other hand, may not have a sump and simply directs surface water directly into the pipe system. Catch basins provide a basic level of stormwater treatment through sedimentation, while simple inlets do not. Some jurisdictions require catch basins specifically because of their pollutant removal capability. The choice between them depends on local regulations, maintenance capacity, and water quality requirements.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy