In short: a 1,000 sq ft paved area at a 4 in/hr design storm needs about 37 GPM of drainage capacity — comfortably inside a 6-inch trench drain channel (about 240 GPM at a 0.5% slope), draining through a 4-inch outlet pipe. Bigger areas, harder surfaces or heavier storms push that number up fast, so run your own figures through the calculator above.
These figures are planning aids based on standard hydraulic formulas. For structural, code-compliance or safety-critical drainage design, confirm with a licensed engineer.
How to size a trench drain
Sizing a trench drain is really two calculations. First, work out how much stormwater the drain has to handle — the Rational Method does this from the contributing area, its surface type, and a design rainfall intensity. Second, check whether the trench drain channel, and its outlet pipe, can actually carry that much flow — Manning's equation answers that from the channel's cross-section, slope and roughness.
Worked example. Take 1,000 sq ft of concrete paving (C = 0.90) in a 4 in/hr design storm. That area is 1,000 / 43,560 = 0.02296 acres. Q = 0.90 × 4 × 0.02296 = 0.0826 cfs, which is 0.0826 × 448.831 = 37.1 GPM. That is the flow the trench drain and its outlet pipe both need to carry.
To check the channel, this calculator applies Manning's equation, Q = (1.486/n) × A × R2/3 × S1/2, to the channel's cross-section at 80% of its rated depth (leaving freeboard), and compares that capacity to the required Q. The same equation, applied to a circular pipe flowing full, sizes the outlet.
Why each input changes the answer
Area and surface type drive demand directly — doubling the paved area, or swapping gravel for concrete, roughly doubles the required capacity because both scale the Rational Method's Q linearly.
Rainfall intensity does the same: a 6 in/hr storm demands 50% more capacity than a 4 in/hr storm on the same area, which is why choosing a realistic local design-storm figure matters more than any other single input.
Channel width increases capacity fast, because a wider channel adds both cross-sectional area and hydraulic radius — going from a 4-inch to a 6-inch channel more than doubles capacity, not just adds 50%.
Slope matters too, but more gently: Manning's equation scales capacity with the square root of slope, so doubling the slope only adds about 41% more capacity. Slope also sets the channel's design velocity, which matters for keeping sediment moving (see below).
Trench drain channel capacity by width
Manning capacity at a 0.5% slope, polymer-concrete channel (n = 0.011), flowing at 80% of the channel's typical rated depth. Confirm exact channel depth with your manufacturer's spec sheet — these are representative figures.
| Channel width | Typical full depth | Capacity (GPM) | Design velocity (ft/s) |
|---|---|---|---|
| 3 in | 3.5 in | 47.0 | 1.79 (below 2 ft/s) |
| 4 in | 4.5 in | 96.7 | 2.15 |
| 6 in (most common) | 5.9 in | 241 | 2.73 |
| 8 in | 7.5 in | 489 | 3.27 |
| 12 in | 9.5 in | 1,161 | 4.08 |
Outlet pipe capacity by diameter
Manning capacity for a smooth plastic pipe (n = 0.011) flowing full at a 0.5% slope. Most codes set a 4-inch minimum for site storm-drain piping, which is why this calculator never recommends smaller.
| Pipe diameter | Capacity (GPM) | Capacity (cfs) |
|---|---|---|
| 4 in (code minimum) | 71.4 | 0.159 |
| 6 in | 211 | 0.469 |
| 8 in | 453 | 1.010 |
| 10 in | 822 | 1.832 |
| 12 in | 1,336 | 2.978 |
Worked examples
Residential driveway apron
37.1 GPM
1,000 sq ft of concrete (C = 0.90) at a 4 in/hr storm needs 37.1 GPM. A 6-inch channel at 0.5% slope (241 GPM capacity, 15% used) is more than adequate, draining through a 4-inch outlet pipe.
Commercial loading dock
167 GPM
3,000 sq ft of asphalt (C = 0.90) at a 6 in/hr storm needs 167 GPM — 69% of a 6-inch channel's 241 GPM capacity. The outlet pipe now needs to step up to 6 inches, since a 4-inch pipe (71 GPM) falls short.
Small patio drain
1.5 GPM
200 sq ft of lawn draining across a patio strip (C = 0.25) at a 3 in/hr storm needs only 1.5 GPM. Even the smallest 3-inch channel (47 GPM) is far oversized here; the outlet still needs a 4-inch pipe to meet the code minimum.
What else affects the result
Self-cleansing velocity. A channel that is technically big enough can still silt up if the water moves too slowly. At the standard 0.5% slope, only the 3-inch channel drops below the commonly recommended 2 ft/s self-cleansing velocity — a slightly steeper slope, or the next channel size up, fixes it.
Manufacturer channel geometry. This calculator uses representative depth-to-width ratios for standard trench drain channels. Actual invert shape, built-in slope and grate open area vary by manufacturer and can shift capacity by 10–20% — always cross-check against the product's published flow-capacity chart before finalizing a spec.
Upstream restrictions. A perfectly sized channel and pipe can still back up if the downstream storm drain or catch basin is undersized. Size the whole run, not just the trench drain, especially for larger commercial sites.
Multiple contributing areas. If several roof downspouts, swales or paved sections feed one trench drain run, add their Rational Method flows together before checking channel capacity.
Frequently asked questions
What size trench drain do I need for a driveway or patio?
For a typical 1,000 sq ft paved driveway apron with a 4 in/hr design storm, the required capacity is only about 37 GPM, well inside what even a 6-inch trench drain channel carries (roughly 240 GPM at a 0.5% slope). Most residential driveways and patios are comfortably served by a 4 to 6-inch channel; larger commercial aprons or loading docks often need 8 inches or more. Run your own area, surface type and rainfall figures through the calculator above for an exact answer.
How is trench drain capacity calculated?
Two steps. First, the Rational Method (Q = C × i × A) estimates how much stormwater the contributing area produces during a design storm. Second, Manning's equation estimates how much flow the trench drain channel — and its outlet pipe — can actually carry at the chosen slope and roughness. The channel is adequately sized when its Manning capacity meets or exceeds the Rational Method demand.
What slope should a trench drain channel have?
A minimum slope of about 0.5% (1/16 in per foot) is common practice for trench drain runs, since Manning's equation shows capacity growing with the square root of slope — doubling the slope only adds about 41% more capacity. Many polymer-concrete channel systems build a fixed internal slope into the channel body itself, so check the manufacturer's spec sheet for the actual invert slope before assuming a flat installation.
Does the outlet pipe need to match the channel width?
No — the outlet pipe only needs enough Manning capacity to carry the required flow, which is usually a smaller diameter than the channel width. A 6-inch channel often drains through a 4-inch outlet pipe. Most plumbing codes also set a 4-inch minimum diameter for site storm-drain piping, so this calculator never recommends anything smaller even when the math would allow it.
What rainfall intensity should I use to size a trench drain?
Use your local design-storm intensity in inches per hour, typically taken from NOAA Atlas 14 for a return period your jurisdiction requires (often 10 or 25 years). A commonly used planning figure in many regions is around 4 in/hr; coastal and high-rainfall areas run higher. When in doubt, use the more conservative (higher) figure your local code or civil engineer specifies.
Can a trench drain be too big?
Yes, in one specific sense: an oversized channel run at a shallow slope can carry water at a very low, sluggish velocity, letting grit and debris settle instead of washing through. Aim for a design velocity of at least 2 ft/s in the channel. If a wider channel drops your velocity below that, either increase the slope slightly or keep the narrower size and clean the channel more often.
Sources & references
- 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.
- Kuichling, E. (1889). The relation between the rainfall and the discharge of sewers in populous districts. Transactions of ASCE, 20, 1–56.
- International Code Council. International Plumbing Code, Chapter 11 — Storm Drainage.
- NOAA 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)