French Drain Rock Calculator: Gravel Cubic Yards, Tons, Filter Fabric, and Cost for Perforated Pipe Drainage Trenches

The gravel needed for a 100-foot French drain depends on the trench width, trench depth, and pipe size. Using the most common residential specifications (12-inch wide trench, 18-inch deep, 4-inch perforated pipe, #57 crushed stone): the trench volume is 100 times (12/12) times (18/12) equals 150 cubic feet. The 4-inch pipe occupies approximately 8.7 cubic feet over 100 feet. The net gravel volume is 150 minus 8.7 equals 141.3 cubic feet, which equals 5.23 cubic yards. At the standard #57 stone density of 1.5 tons per cubic yard, you need approximately 7.85 tons, rounding up to 8 tons for a safety margin. At a typical delivered price of $45 per ton plus $75 delivery, the material cost would be around $435. In bags, 141 cubic feet divided by 0.5 cubic feet per bag equals 282 bags at 0.5 cubic feet each, which at $6 per bag equals $1,692. This makes the bulk delivery option approximately $1,250 cheaper for this project size. You would also need approximately 400 square feet of filter fabric to line the trench (100 feet times a 4-foot wrap perimeter including overlap). For a wider or deeper trench (for example, 18 inches wide and 24 inches deep), the gravel volume increases proportionally, so always enter your specific dimensions into this calculator for an accurate result rather than using this example directly.

#57 crushed stone is the best gravel for French drains and is the standard recommended by drainage engineers, landscape contractors, and the Natural Resources Conservation Service throughout the United States. #57 crushed stone has angular particles ranging from 3/4 inch to 1 inch in size, and the angular shape is critically important: when angular stone pieces are piled together, they rest on their edges and corners rather than flat faces, creating large stable pore spaces between stones that allow water to flow freely and that resist compaction under load. River rock and pea gravel, while visually similar, have smooth rounded surfaces that allow the stones to pack together more tightly, significantly reducing drainage capacity over time and making them poor choices for the main fill layer around a perforated pipe. If you want a decorative surface appearance over the French drain trench, the recommended approach is to use #57 crushed stone for the majority of the trench fill around and above the pipe, then cap the top 2 to 3 inches with river rock or decorative stone for visual appeal. Never use sand, soil, or any fine material within the gravel bed of a French drain. #57 crushed stone is available by the ton from local quarries and landscape supply companies in all US regions and is also sold in bags at major home improvement retailers, though bulk purchasing is far more economical for any project over 2 cubic yards. The #89 crushed stone (3/8 to 1/2 inch size) is an acceptable alternative in soils with very fine clay particles, where the smaller stone size provides better filtration, but it is less widely available and typically costs slightly more per ton.

The correct depth for a French drain depends on what problem you are solving. For surface water management, where the goal is intercepting runoff or redirecting water that collects on the soil surface or just below it, a relatively shallow trench of 12 to 18 inches deep is typically adequate and is the most common residential French drain depth. For subsurface drainage where the goal is lowering a perched water table, relieving hydrostatic pressure against a foundation wall, or intercepting water that is traveling through the soil at depth, the trench must be deep enough to intercept the water where it is actually moving, which may require 24 to 36 inches or more depending on the soil profile. A quick diagnostic: if you see water ponding on the surface and the ground becomes boggy after rain, a shallow 12 to 18 inch drain is usually appropriate. If your basement or crawl space is wet despite surface water being managed, you likely need a deeper drain to intercept subsurface flow. There is also a practical constraint on depth: the outlet point (where the drain daylights and releases water) must be lower than the bottom of the trench at its starting point by at least the accumulated slope drop over the drain length. For example, a 100-foot drain with a 1 percent slope starting at 18 inches deep will have a bottom elevation 1 foot lower at the outlet end, meaning the outlet must be at least 30 inches below grade at the discharge point. The USDA NRCS at nrcs.usda.gov publishes drainage engineering guidance that covers subsurface drain depth selection for different soil types and water table conditions.

The standard residential French drain trench width is 12 inches, which provides adequate space to lay a 4-inch perforated pipe with sufficient gravel coverage on all sides. A 12-inch trench gives approximately 4 inches of gravel on each side of a 4-inch pipe, which is the minimum recommended lateral coverage for proper drainage into the pipe from both sides of the trench. For 6-inch pipe, widening the trench to 14 to 16 inches is appropriate to maintain adequate side coverage. In situations where the drainage area is very large or the water input is very high (such as commercial applications or catching water from multiple roof downspouts), wider trenches of 18 to 24 inches increase the total gravel volume in the trench and provide greater water storage capacity during peak flow events, which helps the system handle surges without backing up. For narrow ornamental trenches or space-constrained situations, an 8-inch trench with a 3-inch pipe is the practical minimum for functional drainage, though this limits water collection capacity significantly. When calculating the correct trench width for your pipe size, add the pipe outer diameter plus a minimum of 4 inches on each side: for a 4-inch pipe (approximately 4.5-inch OD), the minimum trench width is 4.5 plus 4 plus 4 equals approximately 12.5 inches, making the standard 12-inch excavation nearly sufficient and a 14-inch trench comfortably adequate. Wider trenches require more gravel (and fabric) per foot of length, which this calculator accounts for automatically when you enter your actual trench width.

Whether a French drain requires a permit in the United States depends on your local jurisdiction, the scope of the project, and where the drain outlets. Requirements vary significantly between states, counties, and municipalities. In general, simple French drains that remain entirely within your property and outlet to a dry well, infiltration area, or natural low point on your own land typically do not require a permit in most US residential zones. However, French drains that tie into or outlet to a storm sewer, drainage ditch, creek, or any public waterway almost always require permits and may need to meet local stormwater management standards. Drains that cross property lines, outlet onto a neighbor’s property, or affect shared drainage patterns may also require neighbor agreements and municipal review. In areas with specific drainage ordinances (common in flood-prone regions and in newer subdivisions governed by HOA drainage covenants), any significant drainage modification may require HOA approval in addition to or instead of a municipal permit. Always call 811 (the national Call Before You Dig number in the US, available at call811.com) before digging any trench regardless of permit status, as underground utility lines including gas, electric, cable, and water lines can be at residential trench depths and striking them is dangerous and expensive. Local utility notification is a legal requirement in all 50 states. For permit questions, contact your local building and zoning department or county engineering office before starting work. Many jurisdictions also have specific rules about where drainage can outlet, including restrictions on directing drainage to impervious surfaces, neighbor’s property, or wetland areas.

A properly installed French drain with high-quality filter fabric and appropriate gravel can last 30 to 40 years or more before requiring significant maintenance or replacement. The most important factor determining lifespan is whether filter fabric was installed correctly: drains with well-installed non-woven geotextile filter fabric consistently outlast unfiltered drains by 10 to 20 years because the fabric prevents soil migration into the gravel bed that otherwise progressively reduces drainage capacity. Without filter fabric, even high-quality angular gravel like #57 crushed stone will silt up in clay-heavy soils within 5 to 10 years, requiring excavation and replacement of the gravel bed. Other factors affecting French drain lifespan include soil type (sandy soils cause less siltation than clay), tree root intrusion (roots seek out drainage trenches and can block perforated pipes within 5 to 15 years in areas with large trees), quality of the perforated pipe (schedule 40 PVC or HDPE corrugated pipe lasts indefinitely; cheaper thin-wall flexible pipe can collapse under soil loading within 10 to 15 years), and whether the outlet remains functional (an outlet that becomes blocked or submerged will cause the system to back up and ultimately fail). Signs that a French drain is failing include water ponding again in areas that previously drained well, soggy ground over the drain trench even in dry periods, and visible soil contamination of any outlet flow. Periodic flushing of the perforated pipe from the outlet end with a garden hose is a simple maintenance step that can extend drain life by clearing any debris accumulation in the pipe. The USDA NRCS drainage manual at nrcs.usda.gov covers subsurface drain maintenance and service life expectations in detail for agricultural and residential applications.

French drain installation is a manageable DIY project for most homeowners for simple single-trench runs on accessible, flat-to-moderately-sloped ground. The work is physically demanding but does not require specialized skills beyond the ability to dig a straight trench to a consistent depth and slope, follow a level, and handle basic drainage materials. For short runs (under 50 feet), hand digging with a flat spade and mattock is feasible for most physically capable homeowners, though it is slow work, particularly in rocky or heavily rooted soil. For longer runs (50 feet or more) or in hard or rocky soil, renting a trenching machine (available at most Home Depot Tool Rental and United Rentals locations for $150 to $350 per day) dramatically reduces labor time and produces a more consistent trench width and depth than hand digging. Trenching machines are available in different sizes; a 4-inch or 6-inch cut width trencher is appropriate for standard residential French drain work, while an 8-inch or 12-inch cut width unit is needed for wider trenches. The DIY process: mark the trench route with spray paint or string line, confirm utility clearance through 811, excavate the trench to the required depth with consistent slope (verified with a line level or laser level every 10 feet), lay and secure the filter fabric in the trench bottom and sides, set the perforated pipe (holes down or to the sides, not up) on a 2-inch gravel bed, fill the trench with gravel to approximately 6 inches below grade, fold the filter fabric over the top of the gravel, and backfill the top with soil or decorative material. Situations where hiring a contractor is recommended: complex multi-branch drainage systems, proximity to foundations, in areas with extensive tree roots that require careful routing, or in situations where local permits require licensed contractor work.

French drains and dry creek beds both solve drainage problems in residential landscapes but work through different mechanisms and are suited to different situations. A French drain is a subsurface system: the gravel-filled trench and perforated pipe are buried below the soil surface, collecting water that has infiltrated the soil or pooled below the surface and redirecting it underground to an outlet point. The French drain is generally invisible once installed except for a ground-level gravel or sod surface over the trench. A dry creek bed, by contrast, is a surface drainage feature designed to handle overland water flow. It is a visually designed channel filled with rocks and decorative stone that directs surface water across the landscape during rain events, functioning like a natural stream channel, and appears dry between rain events. Dry creek beds are not buried and are intended to be an aesthetic landscape feature as well as a functional drainage channel. The two systems are not mutually exclusive and are often used together: a dry creek bed manages surface runoff visually while a French drain below the creek bed provides additional subsurface drainage capacity and helps the system handle heavy rain events. The choice between them depends primarily on where the water is coming from: if water is pooling on the surface after rain and the problem is overland flow, a dry creek bed is appropriate; if water is appearing from the soil itself, seeping through walls, or coming from a water table issue, a French drain is the right solution. Many homeowners install both in problem drainage areas for a comprehensive solution. The dry creek bed calculator also available on this site can help you size rock quantities for that type of project.

Calculating the slope for a French drain involves measuring the vertical elevation difference between the starting point and the outlet point of the drain, then expressing it as a percentage or as inches of drop per 10 feet of horizontal run. The minimum recommended slope is 1 percent (1 inch per 10 feet, or 1 foot per 100 feet), and 1.5 to 2 percent (1.5 to 2 inches per 10 feet) provides more reliable drainage and is the preferred specification for residential projects. To measure slope in the field without a surveying instrument, the most accessible method for homeowners is the string-and-level method: drive stakes at the start and end points of the proposed trench, run a string between them at a known height above grade, and use a line level (a small bubble level that hangs on the string) to level the string. The height difference between where the string intersects the grade at the start versus the end, measured in inches, divided by the trench length in feet and multiplied by 10, gives you the natural grade in inches per 10 feet. If the existing grade is less than 1 inch per 10 feet, you need to compensate by excavating the trench bottom to an increasing depth toward the outlet end. For a 50-foot drain that needs a 1-inch-per-10-foot slope: the total vertical drop needed is 5 inches over 50 feet; if the surface is flat, the trench bottom must be 5 inches deeper at the outlet end than at the inlet end, which means if you start 18 inches deep, the outlet end must be 23 inches deep. A laser level tool (available for rent from tool rental departments at major home improvement stores) makes this measurement much more accurate than a string level for longer trench runs and irregular terrain.

The outlet is one of the most critical design decisions in a French drain system because a blocked or poorly designed outlet will cause the entire drain to back up and fail. The four main outlet options for residential French drains in the US are: daylighting to a slope or low area, connecting to a storm sewer or municipal drainage system, discharging to a dry well or infiltration basin, and connecting to a rain garden or bioretention area. Daylighting to a slope or open area is the simplest and most reliable option: the drain pipe terminates in an exposed pop-up emitter or open end at the lowest accessible point of the property (a slope, the edge of a yard, near a storm drain at street level, etc.) where water simply runs out onto the ground and disperses. This is the preferred outlet when space allows. Connecting to a municipal storm sewer (the underground pipe system beneath streets in most US communities) requires a permit in almost all jurisdictions and professional installation, but it eliminates the need for an on-property outlet point and is the only feasible option in some urban areas with no suitable grade difference to daylight. A dry well is an underground gravel-filled pit that allows discharged water to infiltrate gradually into the surrounding soil; it is a good option in areas with permeable soil and no suitable outlet grade, but it can become saturated during heavy rain in clay-heavy soils and requires periodic inspection to confirm it is still draining. A rain garden, which is a shallow planted depression designed to hold and infiltrate stormwater, is an increasingly popular eco-friendly outlet option that also provides habitat and aesthetic value. Check with your local municipality through EPA’s local resources at epa.gov/npdes/stormwater for specific outlet requirements in your area, as regulations about surface discharge vary significantly by jurisdiction.

The number of cubic yards in a ton of gravel depends on the type and density of the stone. For common French drain materials: one ton of #57 crushed stone equals approximately 0.667 cubic yards (since #57 has a bulk density of 1.5 tons per cubic yard); one ton of pea gravel equals approximately 0.714 cubic yards (1.4 tons per cubic yard density); one ton of river rock equals approximately 0.74 cubic yards (1.35 tons per cubic yard density); and one ton of crushed limestone equals approximately 0.69 cubic yards (1.45 tons per cubic yard). These conversions are important because landscape supply companies and quarries in the US typically price and sell gravel by the ton for bulk delivery, while this calculator outputs volume in cubic yards. To convert: take the cubic yards needed from this calculator and multiply by the tons-per-cubic-yard density for your chosen gravel type. For example, 3.5 cubic yards of #57 crushed stone equals 3.5 times 1.5 tons per cubic yard equals 5.25 tons to order. When calling a landscape supplier or quarry to order, asking for a specific number of tons is the standard purchasing unit and allows the supplier to determine how many truckloads are needed. A standard 10-wheel dump truck typically carries 12 to 15 tons of gravel, while a smaller single-axle truck carries 5 to 7 tons. Some suppliers also sell by the cubic yard, in which case you can use the cubic yard figure from this calculator directly. Always confirm the selling unit (tons versus cubic yards) and the material weight per cubic yard with your specific supplier before ordering, as actual densities can vary by quarry source.

French drain failures have a predictable set of causes, and most of them are preventable with proper installation technique. The leading cause of failure in residential French drains is siltation of the gravel bed, where fine soil particles (particularly in clay-heavy soils) migrate into the pore spaces of the gravel over time, gradually reducing and eventually eliminating the drainage capacity of the stone. This siltation is almost entirely prevented by installing high-quality non-woven geotextile filter fabric correctly wrapped around the gravel bed. The second most common failure cause is tree root intrusion: the perforated pipe in a French drain is an ideal environment for tree roots, which grow toward moisture sources and can completely fill the pipe interior within 5 to 15 years in areas near trees. Prevention requires routing the drain away from large trees when possible, and using root-resistant slotted HDPE or solid PVC pipe rather than porous sock-wrapped pipe in areas where root intrusion is likely. Outlet blockage is the third common failure mode: if the outlet end of the pipe becomes clogged with sediment, covered by vegetation growth, or blocked by soil slumping, the entire system backs up. Inspect and clear the outlet at least once per year, particularly after storms. Inadequate slope causes the system to work poorly from the start and is not correctable without excavation; always verify adequate grade before digging. Poor gravel choice (using rounded pea gravel or river rock as the main fill rather than angular #57 crushed stone) allows the stones to pack together and reduces drainage over time; this is also not correctable without excavating and replacing the gravel. Finally, undersized pipe for the drainage area served will cause the pipe to flow full during heavy rain events and slow the system’s response to peak flows; for large drainage areas, sizing the pipe to 6 inches or more is important for adequate capacity.

A French drain can solve some wet basement problems but not all of them, and correctly diagnosing the source of the moisture is essential before investing in a drainage solution. If the basement moisture comes from surface water or roof drainage that runs toward the foundation (the most common situation in many US homes), an exterior French drain installed around the perimeter of the foundation, also called a curtain drain, is highly effective at intercepting and redirecting this water before it reaches the foundation wall and basement. This type of drain is typically installed 5 to 10 feet from the foundation at a depth sufficient to intercept the water flow, and it outlets to a lower point on the property or to a storm drain. If the moisture is coming from a high water table that rises to or above the basement floor level during wet periods, an exterior perimeter drain along the footing may help lower the water table near the foundation, but this is a more complex situation that often requires professional assessment to determine whether the water table level can realistically be controlled by a residential drainage system, as this depends heavily on regional hydrology and soil conditions. Interior basement drain systems (sometimes called interior French drains, though they are not technically drains in the French drain sense) collect water that has already entered the basement at the floor-wall joint and pump it out via a sump pump; these address the symptom rather than the source but can be effective when exterior drainage is not feasible. The American Society of Home Inspectors at ashi.org and your state’s cooperative extension water management resources can help homeowners diagnose the specific moisture source and select the appropriate solution for their region and soil conditions.

Filter sock is a mesh tube of fabric or knitted material that slides over perforated drainage pipe to provide an additional layer of filtration at the pipe perforations, preventing fine soil particles from entering the pipe interior while still allowing water to flow in. Perforated pipe comes in two versions: bare pipe (no filter) and socked pipe (factory-installed filter sock already on the pipe). Whether to use filter sock depends on your specific soil conditions and installation method. In sandy or gravelly soils, filter sock is generally not needed because the surrounding gravel provides sufficient filtration and the individual soil particles are too large to pass through the pipe perforations anyway. In clay-heavy soils with very fine particles, filter sock provides an added barrier against pipe clogging, but it has an important limitation: filter sock itself can clog with fine clay particles more quickly than the gravel bed, reducing the amount of water that enters the pipe and defeating the drainage purpose within 3 to 5 years. This is why many drainage engineers in clay-heavy soil regions actually recommend using bare pipe surrounded by filter fabric-wrapped gravel rather than socked pipe: the geotextile fabric lining the trench filters the soil before water reaches the gravel layer, and the gravel provides secondary filtration before water reaches the pipe, making pipe-level sock filtration redundant and potentially counterproductive. When in doubt in moderate clay soils, a bare perforated pipe with high-quality non-woven filter fabric wrapping the trench sides and bottom is the most reliable long-term configuration. In areas with very sandy soils that produce little silt, either approach works well.