In short: for most residential lawns and planting beds, a 4-inch perforated pipe at a 1–2% slope is enough. Larger roof, driveway or hillside areas — or flatter trenches — often need 6 inches or more. The exact size depends on your drainage area, the surface's runoff coefficient, the local rainfall rate and the trench slope, which is what the calculator above works out for you.
These figures are planning aids based on the Rational Method and Manning's equation. For drainage that protects a foundation, a structure, or a neighboring property, confirm the design with a licensed civil engineer and your local code.
How to size a French drain pipe
Sizing a French drain is really two calculations joined together. First you work out how much water the trench must move at its worst moment — the peak flow. Then you find the smallest pipe that can carry that flow at the slope you're able to dig, using Manning's equation for pipe flowing full.
Peak flow comes from the Rational Method, the standard small-site formula used across drainage engineering:
Once Q is known, the calculator solves Manning's equation for the smallest standard pipe diameter that carries that flow at your chosen slope and pipe roughness, then rounds up to the next standard size — 3, 4, 6, 8, 10, 12 or 15 inches.
Worked example. A 1,500 sq ft roof (C = 0.90) in a 4 in/hr design storm drains 0.0344 acres × 0.90 × 4 = 0.124 cfs, or about 55.6 GPM. At a 1% slope through perforated PVC (n = 0.011), Manning's equation calls for a diameter of about 3.2 inches — so you'd round up to a 4-inch pipe, which carries roughly 101 GPM flowing full and moves the water at about 2.6 ft/s. That is the same arithmetic behind every result the calculator above shows.
Why "flowing full"? A French drain's perforated pipe rarely runs completely full — most of the time water also seeps through the surrounding gravel. Sizing as if it must carry the entire peak flow on its own is a deliberate safety margin, the same conservative assumption used for storm sewer design.
Why slope and rainfall rate change the pipe size
Slope does the heavy lifting in Manning's equation — capacity rises with the square root of slope, so doubling the slope only increases capacity by about 41%, but halving it cuts capacity by nearly 30%. A gently sloped trench needs a noticeably bigger pipe to move the same water, which is why the pipe slope calculator is worth checking before you dig: even small grading changes shift the required pipe size.
Rainfall intensity and drainage area move the other side of the equation directly — double the area or the design storm's intensity and peak flow doubles too. That is why a small, steep planting bed and a large, flat roof can both call for the same 4-inch pipe: one is driven by a gentle slope, the other by a big catchment area.
Rational Method runoff coefficients
Typical values for common residential surfaces, matching the options in the calculator above. Higher C means more of the rain becomes runoff instead of soaking into the ground.
| Surface | Typical range | Value used |
|---|---|---|
| Lawn/turf — sandy, flat | 0.10 – 0.20 | 0.15 |
| Lawn/turf — clay or sloped | 0.20 – 0.35 | 0.30 |
| Gravel or compacted soil | 0.40 – 0.60 | 0.50 |
| Roof | 0.85 – 0.95 | 0.90 |
| Pavement or concrete | 0.85 – 0.95 | 0.90 |
Quick reference: maximum area per pipe size
Approximate maximum roof or driveway area (C ≈ 0.90) a pipe can carry at a 3 in/hr design storm, before it flows full, using perforated PVC (n = 0.011). Enter these same numbers into the calculator above to see the exact figures.
| Pipe size | 0.5% slope | 1% slope | 2% slope | 4% slope |
|---|---|---|---|---|
| 4 in | 2,560 sq ft | 3,620 sq ft | 5,120 sq ft | 7,250 sq ft |
| 6 in | 7,560 sq ft | 10,690 sq ft | 15,120 sq ft | 21,400 sq ft |
Worked examples
Draining a 1,500 sq ft roof
4-inch pipe
A 1,500 sq ft roof at a 1% slope produces about 55.6 GPM in a 4 in/hr storm. A 4-inch perforated PVC pipe carries roughly 101 GPM flowing full at 2.6 ft/s — comfortable headroom.
A flat backyard lawn swale
4-inch, low velocity
5,000 sq ft of clay lawn on a gentle 0.5% slope drains about 30.9 GPM. A 4-inch corrugated HDPE pipe just clears it (about 32.7 GPM) but only moves at 0.83 ft/s — below the 2 ft/s self-cleaning threshold, so silt can settle. Steepen the trench where you can.
A small paved driveway apron
3-inch pipe
2,000 sq ft of concrete at a 2% slope also produces 55.6 GPM — the same peak flow as the roof example — but the steeper slope lets a 3-inch perforated PVC pipe handle it, with about 66 GPM of capacity (19% headroom) at 3.0 ft/s.
What else affects the result
Silt and root intrusion. A pipe's effective roughness rises as fine soil, algae or roots build up inside it, cutting real-world capacity well below the clean-pipe figure this calculator reports. Wrapping the pipe and trench in filter fabric is the main defense (see the FAQ below).
Frost depth. In freezing climates the trench needs to sit below the local frost line, which can push the pipe deeper — and therefore flatter — than the slope you planned for on paper. Check your local frost-depth requirement before finalizing the trench profile.
Outlet and backwater. All of this sizing assumes the outlet — daylight, a storm drain, or a dry well — can accept the flow without backing up. If gravity discharge isn't available, the trench instead drains into a sump pit; size that pump with our sump pump sizing calculator.
Infiltration through the gravel. In free-draining soil, some water leaves through the gravel jacket before it ever reaches the pipe, so the pipe itself may see somewhat less than the full peak flow. This calculator ignores that effect for a conservative, worst-case result.
Frequently asked questions
What size pipe do I need for a French drain?
Pipe size depends on how much water the drain must carry, which is set by your drainage area, the local rainfall rate and the surface type feeding it. Most residential French drains around a lawn or planting bed need a 4-inch perforated pipe; larger roof or driveway areas, or flatter trenches, often need 6 inches or more. Use the calculator above with your own area, rainfall rate and slope for an exact recommendation.
What slope should a French drain pipe have?
Aim for at least 1% (about 1/8 inch of drop per foot of pipe), which matches the International Plumbing Code's minimum for 3–6 inch drainage pipe. A 1–2% slope is typical for residential French drains — steep enough to keep water and fine sediment moving, but gentle enough to dig by hand.
How much water can a 4-inch drain pipe handle?
At a 1% slope with smooth perforated PVC, a 4-inch pipe carries roughly 100 gallons per minute flowing full. Capacity drops with a flatter slope or a rougher corrugated pipe, and rises with a steeper slope — the calculator above shows the exact figure for your inputs.
Should French drain pipe be solid or perforated?
Perforated pipe is used along the collection run, where it is meant to accept water through the surrounding gravel. The last section before the pipe exits to daylight, a storm drain or a dry well is usually switched to solid pipe so the water is carried away rather than leaking back out near the outlet.
What's the difference between a French drain and a foundation drain?
A French drain is a general-purpose gravel-and-pipe trench used anywhere in a yard to intercept and redirect surface or subsurface water. A foundation drain is a specific application of the same idea, installed against a foundation footing to relieve hydrostatic pressure and keep a basement or crawl space dry. Sizing follows the same hydraulics; see our foundation drain calculator for that specific case.
Do I need landscape fabric or a filter sock around the pipe?
In most soils, yes. Wrapping the pipe, or lining the trench, with permeable landscape fabric keeps fine soil particles out of the gravel and pipe perforations, which preserves the flow capacity this calculator predicts. Skipping the fabric in silty or clay soils is one of the most common reasons French drains lose capacity and back up within a few years.
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.
- USDA Natural Resources Conservation Service. Urban Hydrology for Small Watersheds (TR-55). National Engineering Manual.
- International Code Council. International Plumbing Code (IPC), minimum slope for horizontal drainage piping. codes.iccsafe.org
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)