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Yard Drainage Fixes

Yard French Drain Sizing Calculator: Pipe Size, Slope & Capacity

A 4-inch perforated pipe at 1 percent slope carries roughly 86 gallons a minute in smooth PVC, or about 56 in corrugated pipe, flowing full — enough for most yard French drains serving a few thousand square feet of roof and lawn in a heavy storm. Go to 6 inches for larger catchments, very flat runs, or downpour design storms. Slope matters more than diameter. This tool works out the water your drain must carry and checks it against both pipe sizes. Materials and quantities live in the gravel & materials calculator; how a yard drain installs is on the yard French drain page.

What drains toward the trench?Only the area that actually reaches the drain — the roof sections whose downspouts feed it, the paving that sheds toward it, the lawn uphill of it.
Lawn soilClay sheds more of the rain as runoff.
Design stormPeak intensity the drain must keep up with.
Pipe material

Design flow reaching the drain

30 GPM roof 20 · paving 0 · lawn 10

Pipe size

A 4-inch pipe is enough

Your catchment sends about 30 GPM to the drain in this storm. A 4-inch corrugated pipe at 1% carries about 56 GPM full, 42 GPM with a margin for a perforated pipe in gravel — you'd be using about 72% of it.

4-inch corrugated

42 GPM usable

56 GPM flowing full at 1% · you'd use 72%

6-inch corrugated

123 GPM usable

164 GPM flowing full at 1% · you'd use 24%

Usable capacity is 75% of the full-pipe figure, because a perforated pipe in a gravel trench is never a clean, full, pressurised conduit. Once the size is settled, get the quantities from the gravel & materials calculator →

What sets the capacity

Pipe diameter and slope set the capacityCross-section comparing a 4-inch and a 6-inch perforated pipe in a gravel trench. Water enters through the gravel from above and the sides; the pipe's carrying capacity grows with diameter and with the slope along the trench, shown by an arrow running downhill toward an outlet.4" pipe6" pipecapacity ∝ diameter^2.7 × √slope — 6" carries ~3× a 4"; doubling slope adds ~40%downhill along the trench to the outlet
Diameter matters, but slope is the lever you usually control. A 4-inch pipe at 2% carries more than a 6-inch pipe at 0.25%, and below 1% neither stays clean.

How the sizing works

Two numbers, compared. Design flow is the water arriving at the trench at the peak of the storm, worked out by the rational method: runoff coefficient × rainfall intensity × area. Roofs shed almost everything (0.95), paving nearly so (0.9), and lawn sheds 15 to 35 percent depending on the soil. Pipe capacity is what the pipe can carry flowing full, from Manning's equation: it rises with diameter to the power 8/3 and with the square root of slope, and smooth PVC (n = 0.013) beats corrugated (n = 0.020) by about half again. The verdict uses 75 percent of full capacity, because a perforated pipe surrounded by gravel is never a clean, full conduit.

Pipe capacity chart

Gallons per minute flowing full, by diameter, material, and slope:

Pipe0.5% slope1% slope2% slope
4-inch corrugated39 GPM56 GPM79 GPM
4-inch smooth PVC61 GPM86 GPM121 GPM
6-inch corrugated116 GPM164 GPM232 GPM
6-inch smooth PVC179 GPM253 GPM357 GPM

Below 1% the pipe silts; the 0.5% column shows why flat runs fail even at 6 inches. Plan on about three-quarters of these figures for a perforated yard drain.

When a bigger pipe is the wrong fix

  • The run is too flat. Under 1 percent, no diameter keeps the pipe clean. Find more fall with a longer route, measure it with the yard slope calculator, or store the water in a dry well instead.
  • There is no outlet. Capacity is meaningless if the pipe ends in the ground. The drain needs daylight, a storm connection, or a sized dry well.
  • The water is roof water. Downspouts belong in a solid pipe straight to the outlet, not in a perforated drain that lets the water back out along the way. Split them off and the French drain shrinks.
  • The problem is groundwater, not runoff. A shallow yard drain intercepts surface and near-surface water. A seasonally high water table wants a deeper interceptor drain sized on seepage, which is a different design and usually a contractor's job.

What this calculator does not tell you

It does not know your real rainfall, whether the trench fabric will silt, how much water the soil itself contributes, or whether the outlet can take the flow. It sizes the pipe for a yard drain intercepting surface and shallow subsurface water. A perimeter drain at the footing, tied to a sump, is sized and built differently — that is the interior French drain covered on our basement site.

Frequently asked questions

What size pipe for a yard French drain?
4-inch perforated pipe for most yard drains. At a 1 percent slope it carries about 56 gallons a minute in corrugated pipe or 86 in smooth PVC flowing full — enough, with a margin, for roughly 2,000 to 4,000 square feet of roof and lawn in a heavy storm. Go to 6 inches for bigger catchments, downpour design storms, or very flat runs.
4-inch or 6-inch French drain?
A 6-inch pipe carries about three times the water of a 4-inch at the same slope, but it needs a wider, deeper trench and more stone. Choose 6-inch when the calculator shows a 4-inch running over about 100 percent of usable capacity, when the run is long and flat, or when the drain also takes several downspouts. Otherwise 4-inch is the better use of money.
How much water can a French drain handle?
It depends on diameter, slope, and pipe material, not on the label. Flowing full at 1 percent: 4-inch corrugated about 56 GPM, 4-inch PVC about 86, 6-inch corrugated about 164, 6-inch PVC about 253. Because a perforated pipe in gravel never runs clean and full, plan on using about three-quarters of those figures.
How much slope does a French drain need?
At least 1 percent — 1 inch of fall per 8 feet, or about 1/8 inch per foot. Below that, water sits in the pipe and silt settles out until it stops working. More slope adds capacity: doubling the grade adds about 40 percent. If you cannot find 1 percent over the route, the answer is a different route or a dry well, not a bigger pipe.
How long can a French drain be?
As long as the slope holds and you can maintain it. Runs over about 150 feet to a single outlet are worth a cleanout at the midpoint, and often work better as two shorter runs to two outlets. A long run also means the outlet is far below the inlet, which is a lot of fall to find in a flat yard.
Corrugated or PVC pipe for a French drain?
Smooth-wall PVC carries about 50 percent more water at the same size and slope, resists crushing, and can be snaked clean. Corrugated is cheaper, bends around obstacles without fittings, and is fine for short, well-sloped runs. For a long or marginal run the extra capacity of PVC is often cheaper than upsizing to 6-inch corrugated.
Do I need two French drains?
When the calculator says 'too much water for one run', or when two separate areas drain in different directions. Splitting roof water into its own line — a solid pipe straight to daylight or a dry well — and leaving the French drain to handle groundwater and lawn runoff is usually the better split than two parallel French drains.

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