Drainage & Earthworks
French Drain Calculator
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Design and estimate your drainage trench. Calculate the volume of drainage stone needed after accounting for pipe displacement, find the required filter fabric area, and get a total material list for your project.
French Drain Calculator
Estimate drain rock, excavation, fabric, pipe length, and slope drop
Results
Drain rock — yd³ (+waste)
2.88 yd³
Excavation — yd³
3.70 yd³
Estimates assume a rectangular trench. Fabric is approximated as bottom + two sides + overlap. Gravel volume subtracts pipe volume when a pipe diameter is entered.
Step-by-Step Calculation
Step 1 — Excavation
50.0 ft × 12.0 in × 24.0 in = 100.00 ft³
Step 2 — Drain rock
trench section 50.0 × 12.0 in × 18.0 in = 75.00 ft³
− pipe π × (4.0 in ÷ 2)² × length = 4.36 ft³
= 70.64 × (1 + 10.0%) = 77.70 ft³
77.7 ft³ ÷ 27 = 2.88 yd³
Step 3 — Filter fabric
width 12.0 in + 2 × 24.0 in + 12.0 in overlap = 72.0 in
× 50.0 × (1 + 10.0%) = 330 ft²
Step 4 — Pipe and fall
pipe 50.0 × (1 + 10.0%) = 55.0 ft
fall 50.0 × 1.00% = 6.0 in
Result — 2.88 yd³ of drain rock
Anatomy of a French Drain
A proper French drain uses perforated pipe at the bottom, surrounded by clean stone, and wrapped entirely in a permeable geotextile envelope.
Step-by-Step Formulas
1. Trench Volume
Measure the length of the drain run and the cross-section of the trench. Trench width is typically 300–450 mm (12–18 in) depending on soil and pipe size.
2. Pipe Displacement
The drainage pipe (usually 100 mm / 4 in perforated pipe) occupies space inside the trench. We subtract its volume from the pipe-bedding zone to avoid over-ordering stone.
3. Drainage Aggregate
The void space around the pipe is filled with clean, washed 20 mm (¾ in) drainage stone. This is your net order volume.
4. Filter Fabric (Geotextile)
To prevent sediment from clogging the stone, the entire trench should be wrapped in non-woven geotextile fabric. Include a 300 mm (12 in) overlap at the top.
Worked Example
Run: 15 m long, 40 cm wide, 60 cm deep. 100 mm (4") perforated pipe. 10% stone waste.
Trench Volume and Slope Drop
Stone volume per 50 ft of trench at common widths and depths, with the fall required at 1% grade:
| Trench (w × d) | Volume per 50 ft (ft³) | yd³ | Drop at 1% over 50 ft |
|---|---|---|---|
| 12 in × 18 in | 75 | 2.78 | 6.0 in |
| 12 in × 24 in | 100 | 3.70 | 6.0 in |
| 18 in × 24 in | 150 | 5.56 | 6.0 in |
| 24 in × 36 in | 300 | 11.11 | 6.0 in |
Drainage That Keeps Working
A French drain is a simple trench, and almost every failure traces to one of three details:
- Slope Moves the Water: A minimum 1% fall — about 1 inch in 8 feet — is what makes the drain flow rather than pond. Without continuous fall it becomes a buried reservoir.
- Washed Stone Keeps Voids Open: Clean stone with no fines preserves the void space water travels through. Crusher run in a drain packs solid and defeats the whole purpose.
- No Fabric Means a Short Life: Without geotextile separating soil from stone, silt migrates into the voids and the drain silts up. A properly wrapped drain lasts decades; an unwrapped one can fail in a few seasons.
Example Calculations
Three scenarios worked through step by step:
Example 1 — Standard Trench
60 ft long, 18 in wide, 24 in deep, 4 in pipe.
Trench volume = 60 × 1.5 × 2 = 180 ft³
Pipe displacement = π × (0.1667)² × 60 = 5.24 ft³
Stone volume = 180 − 5.24 = 174.8 ft³ = 6.47 yd³
Weight = 174.8 × 100 = 17,476 lb = 8.7 tons
Result: about 8.7 tons of washed stone
Example 2 — Fabric and Slope
Same trench, 1% slope, fabric with 12 in overlap.
Fabric width = 1.5 + (2 × 2) + 1 = 6.5 ft
Fabric area = 60 × 6.5 = 390 ft²
Slope drop = 60 × 0.01 = 0.6 ft = 7.2 in
The outlet sits 7.2 in below the start
Result: 390 ft² of geotextile
Example 3 — Shallow Perimeter Drain
100 ft long, 12 in wide, 18 in deep, 4 in pipe.
Trench volume = 100 × 1 × 1.5 = 150 ft³
Pipe displacement = π × 0.1667² × 100 = 8.73 ft³
Stone = 150 − 8.73 = 141.3 ft³ = 5.23 yd³
Weight = 141.3 × 100 = 14,127 lb = 7.1 tons
Slope drop at 1% = 12 in over the run
Drainage Tip
Drainage Tip
Wrap the geotextile all the way over the top of the stone, not just underneath it — a full burrito wrap. Silt washing down from above is what fills the voids and kills a French drain, and fabric that only lines the bottom does nothing to stop it.
Frequently Asked Questions
- What stone should I use for a French drain?
- Use clean, washed stone (no fines) typically 20 mm (¾ in) or 40 mm (1.5 in). Avoid road-base or "crush and run" aggregates.
- Is a French drain pipe installed holes-up or holes-down?
- Holes must face DOWN. Groundwater rises into the pipe from the bottom as the water table reaches the trench level.
- Why do I need geotextile fabric?
- It prevents soil fines from migrating into and clogging the drainage stone, ensuring long-term drainage performance.
- What slope does a French drain need?
- At least 1%, which is about 1 inch of fall per 8 feet of run. More is better where the site allows. Without continuous fall to a proper outlet, the trench holds water instead of moving it.
- What kind of gravel goes in a French drain?
- Clean washed stone with no fines, typically #57 at around 3/4 in. Crusher run and other graded materials contain dust that packs solid, eliminating the void space the drain depends on to carry water.
- Do I need fabric around a French drain?
- Yes. Geotextile separates soil from stone so silt cannot migrate into the voids. Wrap it fully around the stone, overlapping at the top. This single detail is the main difference between a drain that lasts decades and one that fails quickly.
- Should the pipe holes face up or down?
- Down. Perforations at the bottom let water enter as soon as it reaches the pipe invert, which keeps the surrounding water table lower. Holes facing up mean water has to rise to the pipe crown before it can drain away.
- How deep should a French drain be?
- Eighteen to 24 inches for general yard drainage, and down to footing level for a foundation perimeter drain. Depth should be governed by where the water is and where it needs to go, not by a fixed rule.
- Do I need a pipe at all?
- A stone-filled trench alone will drain, but a perforated pipe multiplies capacity enormously and resists silting better. For anything more than a short run, the pipe is inexpensive insurance and well worth including.
- Where should a French drain discharge?
- To daylight at a lower elevation, a dry well, or a storm connection where permitted. It must go somewhere water can actually leave — discharging into a low spot that already floods simply relocates the problem.