Catch Basin Sizing Calculator
A catch basin that’s too small just backs up water at the low point in your driveway or patio. Enter your paved catchment area, a design rainfall, and a runoff coefficient, and this calculator returns total runoff volume and how many basins to install.
1 in is a typical moderate storm; check local rainfall data for your design storm.
Share of rain that actually runs off — about 90–95% for a hard roof, lower for permeable surfaces.
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Sizing tips
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Catchment & rainfall
System profile
Runoff volume in gallons equals catchment area (square feet) multiplied by rainfall depth (inches), multiplied by 0.623 (gallons per square foot per inch of rain), multiplied by your runoff coefficient. Divide that total by the capacity of the basin size you’re using, then round up to the next whole basin.
Catch Basin Sizes and Capacity
Basin size drives both storage capacity and the grate’s load rating. Bigger paved areas or vehicle traffic call for a larger, more heavily rated basin.
| Basin Size | Water Capacity | Best Use |
|---|---|---|
| 12×12 in | 13 gal | Small paved areas, single downspouts |
| 17×17 in | 26 gal | Driveways and patios |
| 24×24 in | 52 gal | Larger paved or low-lying areas |
How to Size a Catch Basin
- Measure the paved catchment area that drains to the low point where the basin will sit.
- Pick a design rainfall depth in inches based on a typical heavy storm for your area.
- Apply a runoff coefficient paved surfaces typically run 90-95%.
- Convert to gallons using the 0.623 gallons-per-square-foot-per-inch factor.
- Divide by basin capacity and round up to the number of basins needed.
Worked example
A 400 sq ft driveway section (20 ft x 20 ft) with a 1 in design storm and 95% runoff works out like this.
- Catchment area400 sq ft
- Design rainfall1 in
- Runoff coefficient95%
- Total runoff volume236.7 gal
- Basin selected17×17 in catch basin (26 gal)
- Basins needed10
- Total cost10 x $140 = $1,400
A Basin Is Only as Good as Its Outlet
Sizing the basin’s storage volume is half the job. It still needs an outlet pipe sized to move water out as fast as it comes in, or it just becomes a small pond during a heavy storm. A standard 4 in drain pipe is common for residential basins feeding a dry well or daylight outlet, but check the flow rate against your catchment area for bigger paved sections. Also match the grate rating to how the area gets used: pedestrian-rated grates are fine for a patio, but a driveway or any area vehicles cross needs a traffic-rated grate and frame, or it will crack under the load.
Common mistakes
- Sizing storage without sizing the outleta basin with plenty of storage but an undersized outlet pipe still floods during heavy rain.
- Wrong grate ratinga pedestrian-rated grate under a driveway will crack under vehicle weight; match the rating to actual traffic.
- Underestimating runoff coefficientsolid paved surfaces run 90% or higher; using a lower number understates the volume the basin has to handle.
- Spacing basins too far aparton a long sloped driveway, one basin at the bottom may not capture water fast enough; multiple basins spread the load.
What this calculator assumes
Adjust your result if your project differs from these.
- Catchment area is treated as an impervious paved surface with a uniform runoff coefficient.
- The 0.623 gallons-per-square-foot-per-inch conversion is the standard rainfall-to-volume factor.
- This is a storage-volume estimate, not a full hydraulic design for outlet pipe sizing.
- Basins are assumed to connect to a functioning outlet pipe or dry well.
- Cost reflects basin unit price only, not pipe, gravel base, or installation labor.
Common questions
What size catch basin do I need for a driveway?
A 17×17 in basin (26 gal) covers most single-driveway installations. Larger paved areas or steep slopes may need a 24×24 in basin or multiple basins.
What's the difference between a catch basin and a dry well?
A catch basin collects and routes water to an outlet pipe or drainage system. A dry well collects water and lets it infiltrate into the soil on-site instead of piping it away.
Do I need more than one catch basin?
For larger paved areas or long slopes, yes. Multiple basins spread across the low points capture water faster than one basin trying to handle the whole area.
What pipe size should run from a catch basin?
A 4 in solid PVC pipe is standard for residential basins. Larger catchment areas or basins may need a 6 in pipe; check local code for sizing requirements.
Do catch basins need cleaning?
Yes. Debris and sediment collect in the basin sump over time and need periodic clearing to keep the outlet from clogging.
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Catch Basin Sizing Calculator Estimate
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materialcalculate.com/tools/catch-basin-sizing-calculator/
Catchment
| Dimensions | — |
| Area | — |
| System | — |
| Rainfall event | — |
| Runoff coefficient | — |
Result
| Water collected | — |
| Units needed | — |
| Estimated total cost | — |
Notes
Estimate generated by materialcalculate.com. Confirm local rainfall data and sizing with a professional for drainage-critical installations.