French Drain Calculator
Plan a french drain from trench dimensions. Enter length in feet, width and depth in inches, pick a perforated pipe size (or gravel-only), and set a waste factor — you get gravel in cubic yards and tons, pipe length, and landscape fabric square footage.
Example: with Trench length (ft) 50 · Trench width (in) 12 · Trench depth (in) 18 · Perforated pipe 4 in pipe (standard) · Waste factor (%) 10 → Gravel needed (with waste): 2.88 cu yd (78 cu ft).
- Gravel weight≈ 4.0 tons at 1.4 tons/cu yd
- Pipe to buy50 ft of 4 in perforated pipe
- Landscape fabric300 sq ft (6.0 ft wide run)
Computed by the calculator below using its default values. Change any input to see your own numbers.
Gravel volume = trench volume − pipe volume; 27 cu ft = 1 cu yd; washed drain rock runs about 1.4 tons per cubic yard.
How a french drain is built
A french drain is a gravel-filled trench with a perforated pipe near the bottom that intercepts groundwater and carries it downhill. Water moves through the washed stone, enters the pipe through its perforations (holes down, per most manufacturers), and flows to daylight, a dry well, or a storm connection. The landscape fabric lines the trench so soil fines cannot migrate in and clog the stone.
Sizing is straightforward volume math: trench length × width × depth, minus the small cylinder the pipe displaces — about 4.4 cu ft per 50 ft of 4-inch pipe. A typical 50-ft drain at 12 in wide and 18 in deep needs just under 3 cubic yards of rock.
Slope, stone, and depth choices
Pitch the pipe at least 1 inch of fall per 10 feet of run (about a 1% grade) — water will not push itself uphill through gravel. Use washed, angular stone in the 3/4-to-1.5-inch range; crusher run and anything with fines will pack tight and stop draining. For surface water, 18–24 inches deep is common; drains protecting a basement or footing must reach footing depth, which is a bigger excavation and often a job for a drainage contractor.
How it’s calculated
Trench volume = length × (width ÷ 12) × (depth ÷ 12) in cubic feet. Pipe volume = π × (diameter ÷ 24)² × length, subtracted when a pipe is selected. Gravel cubic yards = (trench − pipe) ÷ 27, times (1 + waste ÷ 100). Weight uses 1.4 short tons per cubic yard, typical for washed crushed stone (about 2,800 lb/cu yd). Fabric = (2 × depth + 2 × width + 12 in overlap) ÷ 12 × length.
Gravel density varies by stone type (roughly 1.2–1.5 tons/cu yd) and the fabric figure assumes a full trench wrap with a 12 in overlap — buy by the roll and confirm slope and discharge rules with local code.
Common french drain specs
| Item | Common choice | Notes |
|---|---|---|
| Trench width | 9–12 in | Wider moves more water |
| Trench depth | 18–24 in | Footing drains go deeper |
| Slope | 1 in per 10 ft (≈1%) | Minimum working grade |
| Stone | Washed 3/4–1.5 in | No fines; angular or round |
| Pipe | 4 in perforated | PVC or corrugated, holes down |
Common residential drainage practice from contractor and extension guidance; local codes govern discharge points and depths.
Common mistakes
- Ordering by area instead of volume — a 50-ft drain needs cubic yards of stone, and depth is what drives the number.
- Using crusher run or stone with fines, which compacts into a plug instead of a drain path.
- Skipping the fabric wrap, so silt fills the voids in the gravel within a few seasons.
- Laying the pipe flat or uphill — without about 1% of continuous fall the drain becomes a buried bathtub.
Frequently asked questions
How much gravel does a french drain need?
Gravel = trench length × width × depth minus the pipe's volume, converted at 27 cu ft per cubic yard. The default 50 ft × 12 in × 18 in trench with 4 in pipe needs about 2.6 cu yd, or 2.9 with 10% waste — roughly 1.4 cu ft per linear foot.
How many tons is that?
Multiply cubic yards by about 1.4: washed crushed stone weighs near 2,800 lb per cubic yard. The default job's 2.88 cu yd is about 4 tons — most suppliers sell by the ton and deliver.
Do I really need landscape fabric?
Yes, in almost all soils. Non-woven geotextile lets water pass but blocks silt; without it the gravel voids fill with fines and the drain slowly dies. Wrap the trench with about a foot of overlap at the top.
What slope does the pipe need?
At least 1 inch of drop per 10 feet of run, about a 1% grade, continuously toward the outlet. Check it with a string line or level before backfilling — this is the step DIY drains most often get wrong.
When should I call a professional?
When water is entering a basement, pooling against the foundation, or coming off a neighbor's property. Footing-depth drains, sump tie-ins, and discharge-to-street rules involve excavation and code issues where a drainage contractor or engineer earns their fee.