French Drain Calculator
French Drain Calculator
Total linear footage of the trench from inlet to discharge point.
Standard residential trenches are 8–12 inches wide. Wider trenches handle more water volume.
Minimum 18 inches recommended. Go deeper for foundation drainage (24–36 in).
4-inch pipe is standard for most residential applications. Use 6-inch for heavy drainage areas.
Extra material to account for spillage, irregular trench walls, and compaction. 10% is typical.
Soil expands when excavated. Clay expands the most (+30%), affecting skip hire sizing.
Standard corrugated perforated pipe runs $0.75–$2.00/ft depending on type.
Your estimated hourly rate or hired labor cost.
Typical hand-digging rate in average soil. Increase for clay or rocky ground.
Enter Your Trench Dimensions
Fill in the drain length, trench width, depth, and gravel type to see material quantities and total project cost.
Estimate materials, excavation, and total cost for your drainage project
A French drain is one of the most effective ways to redirect unwanted surface water and subsurface moisture away from foundations, lawns, driveways, and crawl spaces. Named after Henry Flagg French, who popularized the concept in the 1850s, a French drain is essentially a gravel-filled trench containing a perforated pipe that channels water toward a safe discharge point. Despite the straightforward concept, accurately estimating the materials required — gravel, pipe, landscape fabric, and fittings — can be surprisingly complex without the right tool. This French Drain Calculator does the heavy lifting for you. Enter your trench dimensions (length, width, and depth), select your gravel type, pipe diameter, and soil conditions, and the calculator instantly computes every quantity you need to order and every cost you should expect to budget. Whether you are a first-time DIYer tackling a soggy backyard or a contractor pricing out a residential foundation drainage system, this tool saves hours of manual calculation and helps you avoid costly under-ordering. The calculator uses industry-standard formulas to determine gravel volume in cubic yards (the standard purchase unit for bulk delivery), subtracting pipe volume displacement for accuracy. It applies your chosen waste factor (defaulting to 10%) to account for irregular trench walls, compaction losses, and spillage during installation. Landscape fabric area is calculated using the full trench perimeter wrap formula with a 15% overlap allowance — the method recommended by drainage contractors. For cost estimation, the tool pre-populates material prices based on your gravel type selection: pea gravel, crushed stone, river rock, and 3/4-inch drainage rock all carry different per-cubic-yard prices and densities. You can override any price with your local supplier quotes. Labor cost is calculated from your input of hours per 10 linear feet and hourly rate, giving you a realistic DIY time budget based on actual project length. One of the most valuable outputs is the DIY vs. professional installation comparison. Professional French drain installation typically runs $25–$50 per linear foot depending on region and soil conditions. The calculator shows you exactly how much you save by doing it yourself — a figure that often exceeds $1,000 on even a modest 50-foot drain. The soil swell factor feature accounts for the reality that excavated soil expands when removed from the ground. Clay soil expands by approximately 30%, sandy soil by 35%, and silty soil by 40%. This matters for skip hire or hauling estimates — knowing your loose excavation volume means you order the right skip size the first time. The trench cross-section diagram updates dynamically with your inputs, giving you a visual reference of gravel layers, pipe position, and landscape fabric wrap. Use the cost breakdown donut chart to see at a glance where your budget is going — gravel typically represents the largest portion of material cost, while labor dominates total project cost for DIY installs. All results can be exported to CSV for inclusion in project bids, permit applications, or material requisition forms. The print function produces a clean summary page suitable for contractor meetings or hardware store shopping trips.
Understanding French Drain Systems
What Is a French Drain?
A French drain is a gravel-filled trench, usually containing a perforated pipe, designed to redirect surface water and groundwater away from a target area. Water enters through the gravel and perforations in the pipe, then flows by gravity toward a lower discharge point such as a storm drain, dry well, or daylight outlet at the edge of a property. French drains are used to solve a wide range of drainage problems: wet basements, waterlogged lawns, pooling on driveways, and foundation moisture infiltration. The trench is typically lined with landscape fabric to prevent soil particles from clogging the gravel over time, significantly extending the drain's functional lifespan. Trench dimensions vary based on the volume of water to be managed and the available slope, but a typical residential French drain is 6–12 inches wide, 18–24 inches deep, and anywhere from 20 to 200 feet long.
How Are Material Quantities Calculated?
The core calculation begins with trench volume: Length × Width (ft) × Depth (ft). Pipe volume — the space the pipe occupies inside the trench — is subtracted using the formula π × (pipe radius in feet)² × length. This gives the net gravel volume in cubic feet, which is divided by 27 to convert to cubic yards. A waste factor (default 10%) is added to account for spillage, compaction, and irregular trench walls. Gravel weight in tons is derived by multiplying cubic yards by the gravel's density (1.35–1.55 tons/yd³ depending on type). Pipe length is calculated using Pythagoras to account for the slight lengthening caused by the minimum recommended slope of 1/8 inch per linear foot. Landscape fabric area equals Length × (2 × Width + 2 × Depth) × 1.15 for the 15% overlap allowance.
Why Accurate Estimating Matters
Under-ordering gravel is the most common and costly mistake in French drain projects. Gravel is sold in cubic yards, and most suppliers charge delivery fees per trip — running short means a second delivery charge that can easily exceed $50–$100 on top of the material cost. Over-ordering wastes money on material you cannot return and creates a disposal problem for excess gravel. Accurate landscape fabric calculation prevents gaps in coverage that allow soil intrusion, which is the leading cause of French drain failure. Getting excavation estimates right matters when hiring equipment — renting a mini-excavator for a half-day versus a full day is often the difference between a profitable and a money-losing project. This calculator prevents all of these issues by computing each quantity precisely from your actual dimensions.
Calculator Limitations and Site-Specific Factors
This calculator provides estimates based on geometric calculations and typical material prices. It does not account for: highly uneven terrain requiring variable-depth trenching, rock or tree root obstructions that increase excavation labor significantly, local permit requirements for drainage work near property lines, utility line location and avoidance, hydraulic capacity calculations (whether the pipe diameter is sufficient for your rainfall intensity and catchment area), or regional material price variations. Always obtain actual supplier quotes for your location, as gravel prices can vary by 50% or more depending on region and delivery distance. For drainage near foundations or in areas with high clay content, consult a licensed drainage contractor or civil engineer before proceeding.
How to Use This Calculator
Enter Trench Dimensions
Start by typing your total drain length in feet, then select trench width and depth from the dropdowns. Use 12 inches wide and 24 inches deep for a standard residential drain. Increase depth to 36 or 48 inches for foundation drainage applications.
Choose Gravel Type and Pipe Size
Select your gravel type — crushed stone or 3/4-inch drainage rock is recommended for best void space and water flow. Choose a 4-inch pipe diameter for typical residential use, or 6 inches for areas with heavy runoff. Toggle the pipe and fabric checkboxes based on your project design.
Set Waste Factor and Soil Type
Adjust the waste factor slider to 10–15% for typical projects, or 15–20% for very irregular trenches. Select your soil type so the calculator can estimate how much the excavated material will expand when removed — important for sizing a skip or planning hauling.
Review Results and Export
Check the material quantities, cost breakdown donut chart, and the DIY vs. professional comparison to understand your total budget and potential savings. Use the Export CSV button to download a detailed line-item estimate, or Print to produce a clean summary for your supplier or contractor.
Frequently Asked Questions
How deep should a French drain be?
The minimum recommended depth for a residential French drain is 18 inches, though 24 inches is considered the standard for reliable year-round performance. For foundation drainage — where the goal is to intercept water before it reaches basement walls or footings — depths of 36 to 48 inches are common. The pipe should sit at the lowest point in the trench, surrounded by at least 6 inches of gravel below it and 6 inches above. Deeper drains intercept groundwater at a lower level and are less likely to be bypassed by heavy rainfall events. Always check local frost depth — in cold climates, drains should be below the frost line to prevent freezing and blockage.
What type of gravel is best for a French drain?
The best gravel for a French drain is 3/4-inch clean crushed stone or angular drainage rock. Angular crushed stone creates large void spaces that allow water to flow freely and does not compact as readily as rounded pea gravel. Pea gravel is acceptable but provides slightly less drainage capacity per cubic yard. River rock works but is more expensive and can shift over time. Avoid limestone in areas with acidic soils as it can dissolve. Do not use pea gravel that contains fines — look for 'washed' or 'clean' gravel with minimal dust. The gravel should be uniform in size; mixed gradations clog faster and reduce permeability. Budget for 3/4-inch #57 crushed stone for the best long-term performance.
Do I need landscape fabric in a French drain?
Yes, landscape fabric (geotextile filter fabric) is strongly recommended for most French drain installations. Without it, soil particles will gradually migrate into the gravel over 3–7 years, clogging the void spaces and causing the drain to fail. The fabric acts as a filter that allows water to pass while blocking soil. For the best results, wrap the entire trench — bottom, sides, and top — with the fabric before filling with gravel and pipe. Use non-woven geotextile rated at 4 oz/yd² or heavier. In very sandy soils with little clay content, some contractors omit the fabric, but this is the exception rather than the rule. The additional cost of fabric is small compared to the labor cost of excavating and reinstalling a failed drain.
How long does a French drain last?
A properly installed French drain with geotextile filter fabric typically lasts 30 to 40 years. Without fabric, or with inferior fabric, the drain may clog within 5 to 10 years as soil particles infiltrate the gravel. Key longevity factors include: using the correct gravel (washed angular crushed stone, not mixed gradations), ensuring proper slope for self-cleaning flow velocity, using a quality filter fabric wrapped fully around the trench, and ensuring the discharge outlet remains clear. Drains installed near trees are at risk of root intrusion and may require inspection every 5 to 7 years. Solid perforated PVC pipe outlasts corrugated plastic pipe by 15 to 20 years in acidic or sandy soils.
What is a reasonable slope for a French drain?
The minimum recommended slope for a French drain is 1/8 inch per linear foot (approximately 0.5% grade). This provides enough gradient for water to flow by gravity toward the outlet without stagnating in the pipe. A 1/4-inch per foot slope (1% grade) is preferred where site conditions allow, as it provides better self-cleaning velocity that flushes sediment through the pipe. For a 50-foot drain, a 1/8-inch slope means the outlet end should be approximately 6 inches lower than the inlet. Insufficient slope is a leading cause of French drain failure — standing water in the pipe fosters root intrusion and sediment buildup. Use a laser level or a builder's level to verify slope before backfilling.
How much does a French drain cost per linear foot?
DIY French drain installation typically costs $5 to $15 per linear foot for materials only (gravel, pipe, fabric, and fittings), depending on trench size and gravel type. When you include labor at a typical DIY rate of $40–$60/hour and estimate 1.5 hours per 10 feet, total DIY cost rises to roughly $15 to $25 per linear foot. Professional installation by a licensed drainage contractor costs $25 to $50 per linear foot for a standard residential drain, and $50 to $100 or more per foot for foundation drainage near basements or in difficult soil conditions. The DIY savings on a 50-foot drain typically range from $500 to $1,500 over professional installation. For projects longer than 100 feet or near structures, consider renting a mini-excavator to save significant labor time.