What is a French Drain?
A French drain (also called a weeping tile, drain tile, rubble drain, or blind drain) is an underground drainage system designed to redirect surface and groundwater away from a specific area. It consists of a sloped trench filled with gravel or rock, typically containing a perforated pipe at the bottom. Water flows into the gravel, through the pipe's perforations, and is safely carried away to a drainage outlet.
French drains are commonly used to prevent water from pooling around foundations, in backyards, under driveways, or on sports fields. Despite the name, they were popularized by Henry French, an American farmer and judge who wrote about the technique in an 1859 farming guide.
How Does a French Drain Work?
The principle is simple: water always flows downhill and takes the path of least resistance. A French drain creates a path of least resistance for groundwater and runoff:
- Surface runoff flows into the top of the gravel-filled trench
- Groundwater seeps laterally through the surrounding soil into the gravel
- Both water types travel down through the permeable gravel toward the perforated pipe
- The pipe carries the collected water downhill to a safe outlet (storm drain, dry well, or daylight)
French Drain Components
- Trench — typically 6–24 inches wide and 18–36 inches deep, sloped at a minimum of 1% grade (1 inch per 8 feet)
- Perforated pipe — 3", 4", or 6" diameter PVC or corrugated pipe with holes for water entry
- Gravel / crushed stone — washed pea gravel or crushed stone fills the trench around and above the pipe
- Filter fabric (geotextile) — wraps the pipe or lines the trench to prevent soil from clogging the gravel
Recommended Minimum Pipe Slopes
| Pipe Diameter | Min. Slope (Imperial) | Min. Slope (Metric) |
|---|---|---|
| 3 inch / 75 mm | 1/8 in per foot (1%) | 1 cm per meter |
| 4 inch / 100 mm | 1/8 in per foot (1%) | 1 cm per meter |
| 6 inch / 150 mm | 1/16 in per foot (0.5%) | 0.5 cm per meter |
How to Use Our French Drain Calculator
Our tool calculates the materials needed for your French drain installation:
- Choose your unit system — Imperial (feet, inches) or Metric (meters, centimeters)
- Enter trench dimensions — length, width, and depth
- Select pipe options — choose whether to include a perforated pipe and its diameter
- Add filter fabric — check the box if you plan to use geotextile lining (recommended)
- Optional: enter material prices — expand the cost section and select your currency to get a full cost estimate
What the Calculator Gives You
- Trench volume — total excavation in cubic yards (or cubic meters)
- Gravel volume — amount of gravel needed (trench minus pipe volume), plus weight in tons and number of 0.5 cu ft bags
- Pipe length — equal to trench length
- Filter fabric area — calculated to wrap the trench sides, bottom, and include 1 ft (30 cm) top overlap with 10% waste
- Cost breakdown — gravel, pipe, fabric, and total cost in 20 world currencies including USD and RUB
Sample Calculation
Project: 50-foot French drain with a 4" perforated pipe
- Trench: 50 ft × 12 in × 24 in = 50 × 1 × 2 = 100 cu ft = 3.70 cu yd
- Pipe volume: π × (0.167 ft)² × 50 ft ≈ 4.36 cu ft = 0.16 cu yd
- Gravel needed: 3.70 − 0.16 ≈ 3.54 cu yd (≈ 200 bags of 0.5 cu ft)
- Filter fabric: (2×2 + 1 + 1) × 50 × 1.1 ≈ 330 sq ft
French Drain Tips
- Always slope your trench toward the outlet — at least 1 inch per 8 feet (1%)
- Use washed pea gravel or ¾" clean crushed stone for best drainage
- Wrap the pipe (or the entire trench) in filter fabric to prevent fine soil from clogging gravel over time
- For high-traffic areas (driveways), use 4" or 6" rigid perforated PVC pipe instead of flexible corrugated pipe
- A pipeless French drain (trench filled only with gravel, no pipe) works for low-flow situations and is cheaper to install
- Check local codes — some areas require permits for drainage work that connects to public storm systems
Frequently Asked Questions
How deep should a French drain be?
Typically 18–36 inches (45–90 cm) deep. The depth depends on your soil type, groundwater level, and what you are protecting. Drains protecting foundations should be at footing depth (often 4–6 feet).
How wide should a French drain trench be?
A standard width is 6–18 inches (15–45 cm). Wider trenches hold more gravel and drain faster but cost more to dig and fill.
Do I need a pipe in my French drain?
A pipe is recommended for most applications because it carries water faster and over longer distances. A pipeless French drain (gravel only) works in small areas with low water flow, like around a garden bed. Our calculator lets you choose either option.
How much gravel do I need for a French drain?
Use our calculator above! Simply enter your trench dimensions, include or exclude the pipe, and it will tell you exactly how much gravel (in cubic yards or cubic meters), its weight, and how many bags to buy.
What currency can I use for cost estimates?
Our French drain cost calculator supports 20 world currencies including US Dollar (USD), Russian Ruble (RUB), Euro, British Pound, Canadian Dollar, Australian Dollar, Japanese Yen, Chinese Yuan, Indian Rupee, Brazilian Real, and more.