Catch Basin Sizing Chart by Drainage Area (Sq Ft)
Look up the right catch basin size by square footage instead of running the formula by hand.
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A chart is often faster than a formula when you already know the square footage you’re draining and just need a size to buy today. The tables below break that lookup down by surface type and rainfall zone. For a precise number that accounts for your exact rainfall intensity and pipe size, run the figures through the Catch Basin Sizing Calculator instead of interpolating between chart rows.
Master Sizing Chart by Surface Type
| Basin Size | Roof/Pavement Area | Gravel Area | Lawn Area |
|---|---|---|---|
| 6×6 in | Up to 120 sq ft | Up to 250 sq ft | Up to 350 sq ft |
| 9×9 in | 120-250 sq ft | 250-550 sq ft | 350-700 sq ft |
| 12×12 in | 250-600 sq ft | 550-1,300 sq ft | 700-1,600 sq ft |
| 18×18 in | 600-1,200 sq ft | 1,300-2,600 sq ft | 1,600-3,200 sq ft |
| 24×24 in | 1,200-2,200 sq ft | 2,600-4,800 sq ft | 3,200-6,000 sq ft |
These ranges assume a moderate rainfall intensity of roughly 2.5-3 in/hr. Areas with heavier storm patterns should shift one column left; drier regions can often shift one column right.
Chart Terms Explained
| Term | What It Means Here |
|---|---|
| Drainage Area | The total surface that sheds water toward this one basin |
| Runoff Coefficient | The share of rainfall that becomes surface runoff rather than soaking in |
| Rainfall Zone | A regional classification based on typical peak storm intensity |
| Combined Area | Two or more surface types feeding a single shared basin |
Why a Chart Isn’t Always Enough
Charts assume an average rainfall intensity and a single surface type, but most real yards combine a roof, a section of lawn, and sometimes a paved walkway all draining to the same low point. When surfaces mix, add each area’s contribution separately rather than reading one row off the chart, since a small paved section can outweigh a much larger lawn area in total runoff.
Formula Behind This Chart
Required Flow Capacity (GPM) = Drainage Area (sq ft) × Rainfall Intensity (in/hr) × Runoff Coefficient × 0.623
Each chart column above was built by solving this formula for area at a fixed basin capacity and a 2.5-3 in/hr rainfall rate.
How to Use the Chart Correctly
- Measure each surface type feeding the basin separately, not the total lot size
- Match each measurement to its own column, not the combined total to one column
- Check your regional rainfall zone below before trusting the default columns
Example 1: Reading the Chart for a Roof Section
- Roof area: 550 sq ft
- Chart column: Roof/Pavement
- Falls between: 250-600 sq ft row
- Chart result: 12×12 in
A 550-square-foot roof section lands near the top of the 12×12-inch range, so a site with unusually heavy storms in that same range would be a candidate for sizing up to 18×18 inches instead.
Example 2: Combining Two Surface Types
- Paved walkway: 150 sq ft
- Adjacent lawn: 900 sq ft
- Paved contribution: 12×12-in tier on its own
- Lawn contribution: 9×9-in tier on its own
- Combined result: 12×12 in (sized to the higher tier, not added together)
When two surfaces share a basin, size to whichever surface pushes the requirement highest, then confirm with the calculator that the combined flow still fits inside that basin’s rated GPM.
Example 3: Gravel Driveway Section
- Gravel area: 2,000 sq ft
- Chart column: Gravel
- Falls between: 1,300-2,600 sq ft row
- Chart result: 18×18 in
Gravel drains better than solid pavement but still sheds more water than lawn during a hard rain, which is why it sits between the two on every chart row.
Skip the column-matching and get an exact GPM figure for your site.
Use the Catch Basin Sizing CalculatorRainfall Zone Adjustment Chart
| Rainfall Intensity | Typical Region | Chart Adjustment |
|---|---|---|
| Under 2 in/hr | Arid or low-rainfall areas | Shift one basin tier smaller |
| 2-3.5 in/hr | Most temperate US regions | Use chart as-is |
| 3.5-5 in/hr | Gulf Coast, subtropical zones | Shift one basin tier larger |
| Over 5 in/hr | Tropical or extreme storm zones | Shift two tiers larger or add a second basin |
Alternative Approach: Sizing by Existing Pipe Run
If a drain pipe is already installed and only the basin needs choosing, match the basin to the pipe’s rated flow instead of the square footage chart. A 4-inch pipe run tops out near 12×12-inch basin capacity, while a 6-inch run supports an 18×18-inch basin comfortably. This approach works as a shortcut, though it skips the safety check that comparing against actual drainage area provides.
Common Chart-Reading Mistakes
- Adding multiple surface areas together and reading one combined row instead of sizing to the highest individual tier
- Using the roof/pavement column for a mixed gravel driveway
- Ignoring the rainfall zone adjustment and applying the default chart everywhere
- Rounding down to save money when the measured area sits at the top of a tier
- Treating chart results as final without checking outlet pipe compatibility
Chart Lookup vs. Calculator: Which to Trust
A chart is fast and fine for a single, uniform surface in an average rainfall zone. A calculator becomes the better choice once multiple surface types feed one basin, the site sits in a heavier rainfall zone, or the project needs a documented number for a permit or contractor quote. Use the chart to get a ballpark before shopping, then confirm with the calculator before finalizing an order.
Basin Size to Grate Opening Reference
| Basin Size | Typical Grate Opening |
|---|---|
| 6×6 in | ~30 sq in inlet area |
| 9×9 in | ~65 sq in inlet area |
| 12×12 in | ~120 sq in inlet area |
| 18×18 in | ~280 sq in inlet area |
| 24×24 in | ~500 sq in inlet area |
Factors That Shift a Chart Result
- Local rainfall intensity outside the chart’s assumed 2.5-3 in/hr range
- Multiple surface types converging on one basin
- Slope steepness, which concentrates runoff faster than flat ground
- Existing pipe diameter limiting the basin’s effective throughput
- Sediment or debris load that reduces effective sump capacity over time
Chart Lookup Steps Summary
- Measure each surface type feeding the basin separately
- Match each area to its correct chart column
- Identify your rainfall zone and adjust the tier if needed
- Size to the highest tier among combined surfaces, not the sum
- Cross-check the result against outlet pipe compatibility
- Confirm with the calculator before ordering
Frequently Asked Questions
What catch basin size fits a 500 sq ft driveway?
A 500-square-foot paved driveway falls in the 12×12-inch tier under average rainfall, moving up to 18×18 inches in heavier rainfall zones.
Do gravel and lawn areas use the same chart column?
No. Gravel sheds more water than lawn, so it moves to the next basin size sooner even at the same square footage.
Should I add two surface areas together before using the chart?
No. Look up each surface type separately and size to whichever one requires the larger basin, rather than summing the square footage.
How much does rainfall zone change the chart result?
Moving from an average rainfall zone to a heavy one can push a result up by a full basin tier, so always check the adjustment table for your region.
Is a chart accurate enough for a permit application?
Most permitting offices expect a calculated flow rate rather than a chart estimate, so run the exact numbers through the calculator before submitting.
What happens if my area falls right between two chart tiers?
Round up to the larger tier when the measured area sits near the top of a range, especially in heavier rainfall zones.
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