What Is a French Drain and How It Works
A French drain is a subsurface drainage system made of a sloped trench filled with gravel and containing a perforated pipe at the bottom. Water enters the trench through the gravel, flows into the pipe through small perforations, and is carried away to a discharge point like a dry well, storm drain, or daylight outlet. The system relies on gravity and a proper slope to function, typically dropping at least 1 inch for every 8 feet of horizontal run.
Henry Flagg French, a farmer and lawyer from Massachusetts, described this drainage method in his 1859 book on farm drainage, which is how the system got its name. The original design used only gravel and no pipe, but modern French drains almost always include a perforated pipe to handle higher water volumes. Today, French drains are one of the most common solutions for foundation water issues, yard flooding, and retaining wall drainage.
The key to a working French drain is the void space between gravel pieces. Water flows through these gaps and into the pipe, which acts as a high-speed channel. Without enough gravel or the right stone size, the system backs up and water sits in the trench. Getting the material quantities right from the start saves you from having to dig the trench back up later.
How Much Gravel Does a French Drain Need
The gravel volume for a French drain equals the total trench volume minus the volume taken up by the perforated pipe. For a standard residential trench of 50 feet long, 1 foot wide, and 1.5 feet deep with a 4-inch pipe, the gravel requirement works out to roughly 2.5 cubic yards. Ordering by the cubic yard is standard practice at most quarries and landscape supply yards.
Always add 10 to 15 percent to your calculated gravel amount. Trench bottoms are rarely perfectly flat, sidewalls collapse slightly during backfilling, and gravel settles over time. If your calculator says 3 cubic yards, order 3.5 to be safe. Running short mid-project means a second delivery charge and a delay while you wait. You can use our gravel calculator to cross-check the numbers and convert between tons and cubic yards.
The type of gravel matters as much as the quantity. Number 57 crushed stone is the go-to for French drains because the angular pieces interlock and create stable void space. Washed river rock also works well in clay-heavy soils where you want maximum water flow. Before ordering, check whether your supplier sells by the ton or cubic yard — the conversion depends on stone density, which varies by rock type. For reference, one cubic yard of #57 stone weighs approximately 2,800 pounds.
Choosing the Right Pipe for Your French Drain
The perforated pipe is what separates a modern French drain from a basic rubble trench. The two main options are corrugated HDPE (high-density polyethylene) and rigid PVC with drilled holes. Corrugated pipe is flexible, cheap, and easy to install around curves, but it can crush under heavy loads if not buried deep enough. Rigid PVC handles higher loads and resists crushing, making it the better choice under driveways or areas with vehicle traffic.
Pipe diameter affects flow capacity significantly. A 4-inch pipe — the residential standard — can carry about 90 gallons per minute at a 1 percent slope. A 6-inch pipe moves roughly 220 gallons per minute under the same conditions. Most homes need only 4 inches, but if you are draining a large property or deal with heavy storm runoff, upsizing to 6 inches is worth the cost. Our pipe calculator helps you estimate flow rates for different pipe sizes.
Pipe placement in the trench also matters. The pipe should sit at the bottom of the trench with perforations facing down (for corrugated pipe with holes on one side) so water enters from below as it rises through the gravel. Some contractors wrap the pipe in a fabric sock before placing it in the trench, adding an extra layer of filtration on top of the geotextile lining.
French Drain Depth and Width Guidelines
Trench dimensions directly determine how much material you need and how well the drain performs. Width between 9 and 12 inches is standard for residential French drains — wide enough for a 4-inch pipe plus gravel on both sides but narrow enough to keep excavation manageable. Going wider increases gravel cost without meaningfully improving drainage, so stick to 12 inches unless you are running multiple pipes or using a wider pipe.
Depth depends on your drainage goal. To intercept surface water and keep a yard from getting soggy, 12 to 18 inches is usually enough. To protect a foundation or basement, the trench bottom should sit at or below the footing level, which often means 24 to 36 inches deep. In cold climates, the pipe should sit below the frost line to prevent freeze-thaw cycles from cracking it over winter. Always verify with local building codes before digging.
When planning your trench, use the excavation calculator to estimate the total dirt you will pull out of the ground. A 50-foot trench at 1 by 1.5 feet generates about 2.8 cubic yards of spoil — that is roughly 200 wheelbarrow loads. Having a plan for where that soil goes before you start digging keeps the project moving and avoids having a mud pile sitting in your yard for weeks.
Filter Fabric and Lining the Trench
Geotextile fabric is the unsung hero of a long-lasting French drain. Without it, fine soil particles wash into the gravel and pipe, gradually clogging the system until water stops flowing. A good filter fabric lets water through but holds back silt and clay particles, extending the drain's functional life from 10 to 15 years up to 30 or 40 years.
Install the fabric by lining the trench before adding gravel. Roll the fabric along the bottom and up both sidewalls, leaving enough overhang at the top to fold over the gravel once the trench is filled. Use non-woven geotextile fabric for French drains — it has a felt-like texture that filters well and does not tear easily. Woven fabric is stronger but filters poorly and is better suited for stabilization under roads and patios.
Some soils clog faster than others. Sandy soil drains well and is relatively forgiving if you skip the fabric, though you should still use it. Clay soil is the worst offender — the tiny particles suspend in water and travel deep into the gravel before settling. If you have heavy clay, consider a two-layer system with fabric lining the trench and a fabric sock around the pipe itself. You can also use crushed stone as a filter layer between the native soil and the gravel to reduce clogging.
Calculating Soil Removal and Disposal
Every cubic foot of trench you dig produces a cubic foot of soil that needs to go somewhere. Spoil volume is typically 10 to 20 percent larger than trench volume because excavated soil is looser than compacted ground. A 50-foot trench producing 75 cubic feet of soil becomes roughly 90 cubic feet when dug up — that is about 3.3 cubic yards of loose material sitting on your lawn.
You can reuse clean topsoil elsewhere on the property for grading, garden beds, or filling low spots. Subsoil, especially clay, is harder to use productively and may need to be hauled away. If your trench goes through contaminated soil or you suspect chemical runoff from pesticides or industrial activity, contact a local environmental office for guidance on disposal. For clean fill, many communities have free exchange programs where homeowners and contractors trade excess soil.
To convert your excavation volume into hauling loads, divide the total cubic yards by the truck capacity. A standard pickup truck holds about 1.5 cubic yards of soil per load. A dump trailer holds 6 to 10 cubic yards. Use the cubic yard calculator to convert your trench dimensions into the units your hauling service or rental company expects.
French Drain vs Other Drainage Solutions
French drains are not the only option for water problems, and sometimes a different approach works better. Surface drains (also called catch basins) collect water at ground level through a grate and pipe it underground — these are better for handling runoff from patios, driveways, and large flat areas where water sheets across the surface. A French drain handles subsurface water and groundwater seepage, which surface drains cannot address.
Dry wells are another alternative. A dry well is a large underground container surrounded by gravel that collects runoff and lets it percolate slowly into the surrounding soil. Dry wells work well at the end of a French drain line as a discharge point in areas where you cannot route water to a street or natural drainage way. They can also be used standalone for small drainage areas like a single downspout.
For foundation waterproofing specifically, an exterior French drain installed at footing level during construction is the gold standard. Retrofitting one around an existing house requires excavation along the foundation wall, which is expensive but highly effective. If you have a finished basement with water issues, compare the cost of an exterior French drain against an interior perimeter drain installed under the basement slab. The interior version costs less and is less disruptive to landscaping.
Cost Factors and Budgeting for a French Drain
Material costs for a DIY French drain run $4 to $8 per linear foot for the gravel, pipe, and fabric. A 50-foot trench costs roughly $200 to $400 in materials. Professional installation runs $20 to $50 per linear foot including labor, equipment, and material markup. A contractor-installed 100-foot French drain typically costs $2,000 to $5,000 depending on site access, soil conditions, and whether existing landscaping needs removal.
Gravel is usually the largest material expense. Buying in bulk from a quarry or landscape supply yard is significantly cheaper than bagged gravel from a big-box store. A bulk cubic yard of #57 stone costs $30 to $50 delivered, while the equivalent in bags runs $100 to $150. If your trench requires more than half a cubic yard, bulk delivery almost always wins on price. Factor in delivery fees, which range from $50 to $150 depending on distance.
Do not forget the hidden costs: sod replacement over the trench, landscape restoration, and potential utility location fees. If you are digging near a property line or under a sidewalk, you may also need a permit. Before buying materials, use the soil calculator to estimate how much backfill or topsoil you will need to restore the surface after the gravel and pipe go in. For sandy soils that need amending, a layer of sand under the topsoil can improve surface drainage above the trench.