In short: a catch basin's outlet needs to carry whatever flow the Rational Method predicts for its catchment — Q = C × i × A. For a typical 5,000 sq ft paved area in a 4 in/hr design storm, that is about 185 GPM (0.41 cfs), which calls for at least a 6-inch outlet pipe at a 1% slope. Larger, more impervious catchments or heavier storms need proportionally larger pipe.
These figures are planning aids based on standard hydrology methods. For municipal approval or structural work, confirm sizing with a licensed civil engineer and your local drainage code.
How to calculate catch basin sizing
Catch basin sizing comes down to two linked questions: how much stormwater will actually reach the basin during a design storm, and can the outlet pipe carry that water away fast enough to keep it from backing up. The first question is answered with the Rational Method, the standard approach for sizing storm inlets and small drainage structures:
The second question — will the outlet keep up — is answered with Manning's equation, which relates a pipe's diameter, slope and roughness to how much water it can carry flowing full. This calculator solves Manning's equation for the smallest standard pipe diameter whose full-flow capacity meets or exceeds the Rational Method peak flow, then reports the velocity that flow would produce in that pipe.
Worked example. Take a 5,000 sq ft asphalt parking area (C = 0.90) in a 4 in/hr design storm. The area is 5,000 ÷ 43,560 = 0.1148 acres. Peak flow is Q = 0.90 × 4 × 0.1148 = 0.413 cfs, which is 185.5 GPM. Sizing a concrete outlet pipe at 1% slope with Manning's equation calls for at least 5.35 inches of diameter — round up to the next standard size, a 6-inch pipe, which carries 251.8 GPM flowing full and moves this design flow at about 2.10 ft/s.
Always round up, never down. If your required diameter falls between two standard sizes, use the larger one. Undersizing the outlet defeats the purpose of sizing the basin correctly in the first place — water simply backs up at the pipe instead of the grate.
Why area, surface and rainfall change the answer
All three Rational Method inputs move the result in a straight line, but they represent different things on site. Area is simply how much ground drains to this one basin — bigger catchments concentrate more water at a single point, which is why large lots are often split across several basins rather than one oversized one. Surface type sets the runoff coefficient C, which describes how much rain becomes surface runoff instead of soaking into the ground: a roof or paved lot sheds 90–95% of what falls on it, while turf on permeable soil absorbs much of a storm before it ever reaches the basin. If a basin is collecting roof runoff routed through a gutter and downspout system, size those separately for the roof's own peak flow before this basin's outlet.
Rainfall intensity is the input most often guessed rather than looked up, and it deserves care — it is not the total rainfall for a storm but the peak rate over a short design duration (commonly 5 to 10 minutes), because that peak, not the storm's daily total, is what the basin must handle at the moment of highest inflow. Local intensity-duration-frequency (IDF) curves, built from NOAA Atlas 14 rainfall data, give the correct value for your jurisdiction's required return period; the presets here are general planning ranges only.
Typical runoff coefficients (Rational Method)
Representative C values this calculator uses for each surface preset. Published ranges vary by source and site condition — use the midpoint shown, or enter a custom value if you have a site-specific figure.
| Surface | Typical range | Calculator uses |
|---|---|---|
| Roof (impervious) | 0.90 – 0.95 | 0.95 |
| Asphalt / concrete pavement | 0.85 – 0.95 | 0.90 |
| Compacted gravel | 0.50 – 0.70 | 0.55 |
| Lawn / turf, flat (<2% slope) | 0.10 – 0.20 | 0.20 |
| Lawn / turf, steep (>7% slope) | 0.25 – 0.35 | 0.35 |
Standard outlet pipe capacity (Manning's equation, flowing full)
Full-flow capacity of common storm drain pipe sizes at a 1% slope, using Manning's equation. Capacity scales with the square root of slope, so doubling the slope raises capacity by about 41%; halving it cuts capacity by about 29%.
| Diameter | PVC (n = 0.009) | Concrete (n = 0.013) |
|---|---|---|
| 4 in | 123.4 GPM | 85.4 GPM |
| 6 in | 363.8 GPM | 251.8 GPM |
| 8 in | 783.4 GPM | 542.4 GPM |
| 10 in | 1,420.4 GPM | 983.4 GPM |
| 12 in | 2,309.8 GPM | 1,599.1 GPM |
| 15 in | 4,188.0 GPM | 2,899.4 GPM |
| 18 in | 6,810.1 GPM | 4,714.7 GPM |
| 24 in | 14,666.4 GPM | 10,153.6 GPM |
Worked examples
Roof leader to basin
47.0 GPM
A 1,200 sq ft roof (C = 0.95) in a 4 in/hr storm sends about 47 GPM to its catch basin. That only needs a 4-inch outlet — but at 1.20 ft/s, the flow runs slower than the range most codes recommend for keeping sediment moving, so a steeper slope helps here more than a bigger pipe.
Driveway / small parking area
371 GPM
8,000 sq ft of asphalt (C = 0.90) in a 5 in/hr storm produces about 371 GPM. An 8-inch pipe at 1% slope carries 542 GPM flowing full, with the design flow moving at a healthy 2.37 ft/s.
Parking lot
1,391 GPM
A 25,000 sq ft lot (C = 0.90) in a 6 in/hr intense storm reaches about 1,391 GPM. That calls for a 12-inch outlet, which carries 1,599 GPM flowing full at 3.95 ft/s — comfortably within the self-cleansing range.
What else affects the result
Grate and sump, not just the pipe. This calculator sizes the outlet pipe, which is usually the limiting factor — but the grate opening and sump depth also need to handle debris and short-term ponding. A basin with a generously sized grate and adequate sump still backs up if the outlet pipe is undersized, which is why pipe capacity is the calculation this tool focuses on.
Full-flow assumption. Manning's equation here assumes the pipe runs completely full, which gives a conservative (slightly higher) capacity estimate than pipes commonly run in practice, where some freeboard is preferred. Treat the reported capacity and velocity as a sizing check, not a guarantee of the exact flow depth on site. If velocity comes out too low, increasing the pipe slope is often the easiest fix — see the pipe slope calculator for the drop-over-run math.
Debris and clogging. Leaves, sediment and trash reduce a grate's effective open area over time, sometimes significantly. Routine maintenance matters as much as correct sizing — an undersized basin that's kept clean can outperform an oversized one that isn't.
Multiple basins and connected systems. Large or irregular sites are often better served by several smaller, well-placed basins feeding a shared storm drain than by one basin sized for the whole catchment. If your required pipe size looks unreasonably large, that's usually the sign to split the drainage area.
Frequently asked questions
How do you size a catch basin?
Start with the peak stormwater flow the catch basin must handle, using the Rational Method: Q = C × i × A, where C is the runoff coefficient of the catchment surface, i is the design rainfall intensity, and A is the drainage area. That peak flow then sets the minimum size of the outlet pipe leaving the basin, sized with Manning's equation so the pipe can carry the flow at your chosen slope and material.
What is the Rational Method formula for catch basins?
In US customary units, Q (cfs) = C × i (in/hr) × A (acres). C is a dimensionless runoff coefficient describing how much rain becomes runoff versus soaking in, i is the design storm's rainfall intensity, and A is the catchment area draining to the basin. It is the standard method for sizing storm inlets and small drainage structures.
What rainfall intensity should I use?
Use the intensity for the design storm return period your local code requires, typically a 10-year storm over a short duration of 5 to 10 minutes, taken from your local intensity-duration-frequency (IDF) curve or NOAA Atlas 14. The calculator's presets (1 to 8 in/hr) are general planning ranges, not a substitute for local rainfall data.
Why does the outlet pipe matter for catch basin sizing?
A catch basin is only as effective as the pipe carrying water away from it. If the outlet pipe is undersized, water backs up in the basin during storms regardless of how large the grate or sump is. This calculator sizes the outlet pipe with Manning's equation so its full-flow capacity meets or exceeds the Rational Method peak flow.
What if my required pipe size is larger than a standard size?
If the calculated diameter falls between two standard sizes, always round up to the next standard pipe size, never down. If the required size exceeds the largest standard storm pipe (36 inches), the site likely needs multiple basins, a steeper pipe slope, or a review by a civil engineer rather than a single oversized pipe.
Sources & references
- Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of the American Society of Civil Engineers, 20, 1–56.
- Manning, R. (1891). On the flow of water in open channels and pipes. Transactions of the Institution of Civil Engineers of Ireland, 20, 161–207.
- Federal Highway Administration. Urban Drainage Design Manual, Hydraulic Engineering Circular No. 22 (HEC-22).
- NOAA National Weather Service. Atlas 14, Precipitation-Frequency Atlas of the United States.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)