French Drain Calculator

Calculate Trench Gravel, Drain Pipe & Costs

Tired of wet spots or standing water in your lawn? A French drain is an effective, time-tested drainage solution. This calculator helps you determine exactly how much crushed stone, perforated pipe, and landscape filter fabric you need for a successful install.

Drain Length (ft):
Trench Width (in):
Trench Depth (in):
Pipe Diameter (in):	4 inches 6 inches No Pipe (French trench)
Gravel Price ($/ton):
Pipe Cost ($/ft):

What Is a French Drain and Why Does It Matter?

A French drain is a highly effective, time-tested subsurface drainage system designed to intercept and redirect excess surface water and groundwater away from home foundations, retaining walls, lawns, and low-lying garden zones. By utilizing the path of least resistance, a French drain gathers water that saturated soil cannot quickly absorb and guides it safely through a gravel-filled trench to a designated discharge point, such as a dry well, rain garden, street storm sewer, or natural swale. Named after Henry Flagg French, a Massachusetts judge and agriculturist who popularized the system in the mid-19th century, this drainage method is a cornerstone of modern residential and commercial civil engineering, preventing water damage, basement flooding, and soil erosion.

Understanding the physics behind a French drain is essential for proper design. Ground saturation occurs when stormwater fills the pore spaces between soil particles, raising the local water table. When saturated soil is adjacent to a foundation, it exerts hydrostatic pressure against the concrete walls. This pressure forces moisture through microscopic pores and cracks, resulting in wet basements and structural deterioration. A French drain acts as a relief valve: because water moves through crushed gravel significantly faster than through native clay or loam soils, the water table is lowered along the trench. Geotextile filter fabric serves as a barrier, letting water pass while stopping soil sediment from filling the voids in the gravel, which would otherwise clog the drain and render it useless.

Sizing and estimating materials for a French drain trench is a crucial phase of site development. Trench dimensions directly govern the drainage capacity and structural life of the system. A trench that is too shallow or narrow will fail to capture the required volume of groundwater, leading to saturated lawns and surface pooling. Conversely, digging a trench that is wider than necessary dramatically increases excavation labor and the volume of crushed stone required, swelling the project budget. By using precise volume calculations that subtract the pipe volume from the trench space and determine the surface area of the geotextile wrapper, you can ensure a highly functional drainage system while optimizing material costs.

How to Calculate French Drain (Mathematical Formulas)

Estimating French drain materials requires calculating the physical volume of the trench, subtracting the volume occupied by the perforated pipe (to find the net gravel volume), and calculating the perimeter of the trench cross-section to determine the square footage of landscape fabric needed.

Formula for Trench Volume

The total volume of the trench is calculated using simple rectangular geometry, converting all dimensions to feet:

1. Trench Width in Feet (ft): Width (ft) = Trench Width (in) / 12
2. Trench Depth in Feet (ft): Depth (ft) = Trench Depth (in) / 12
3. Total Trench Volume (cu ft): Trench Volume = Drain Length (ft) × Width (ft) × Depth (ft)

Formula for Pipe Volume

If a perforated drain pipe is installed in the trench, it displaces a portion of the gravel. The volume of the pipe is calculated using the cylinder volume formula:

1. Pipe Radius in Feet (ft): Pipe Radius = (Pipe Diameter (in) / 2) / 12
2. Pipe Volume (cu ft): Pipe Volume = π × Pipe Radius² × Drain Length (ft)

Formula for Net Gravel Weight

To find the amount of gravel required, subtract the pipe volume from the total trench volume, and convert the result from cubic feet to cubic yards and tons:

1. Net Gravel Volume (cu ft): Gravel Volume = Trench Volume - Pipe Volume (If no pipe is used, the gravel volume equals the trench volume).
2. Net Gravel Volume (cu yd): Gravel Volume (cu yd) = Gravel Volume (cu ft) / 27
3. Gravel Weight in Tons (tons): Gravel Weight = Gravel Volume (cu yd) × 1.4 (assuming washed crushed stone weighs 1.4 tons per cubic yard, or roughly 2,800 lbs).

Formula for Filter Fabric Area

The landscape fabric must wrap completely around the gravel trench with a 12-inch overlap at the top to prevent soil intrusion. The perimeter of the fabric envelope is calculated as:

1. Fabric Envelope Perimeter (ft): Fabric Perimeter = (Trench Width (in) + (2 × Trench Depth (in)) + 12) / 12
2. Total Fabric Area (sq ft): Fabric Area = Fabric Perimeter × Drain Length (ft)

French Drain Sizing & Specifications Reference Chart

The table below provides a material estimation reference for a 100-foot-long French drain trench using standard washed 3/4" crushed gravel and a 4-inch perforated PVC pipe, assuming a gravel density multiplier of 1.4 tons per cubic yard.

Trench Dimensions (W × D)	Total Trench Vol (Cu Ft)	Gravel Weight Needed (Tons)	Filter Fabric Needed (Sq Ft)	Best Application
8" Width × 12" Depth	66.7 cu ft	2.1 Tons	360 sq ft	Shallow garden beds, minor surface runoff
12" Width × 18" Depth	150.0 cu ft	5.2 Tons	500 sq ft	Residential yard drainage, slope bases
12" Width × 24" Depth	200.0 cu ft	7.2 Tons	600 sq ft	Near patio boundaries, crawl space exterior
18" Width × 24" Depth	300.0 cu ft	11.1 Tons	700 sq ft	Footing drains, high water table interception
24" Width × 24" Depth	400.0 cu ft	15.0 Tons	800 sq ft	Commercial grading, heavy stormwater channels

Step-by-Step Installation Guide & Professional Tips

A French drain is a labor-intensive project, but proper execution ensures a drainage solution that will last for decades. Skip steps or use the wrong materials, and the drain will quickly clog with soil, requiring complete re-excavation. Follow this step-by-step installation guide.

Step 1: Utility Location and Site Planning

Before digging, contact your local utility location hotline (e.g., 811 in the U.S.) to mark any underground power, gas, water, or telecommunication lines. Plan the path of the trench from the wet catchment area to a safe discharge point. Establish the slope: the trench must slope downhill at a rate of at least 1% (1/8 inch per foot, or 1 foot of drop per 100 feet of run) to allow water to drain by gravity.

Step 2: Excavation

Dig the trench along the marked path. For residential yard drainage, a width of 12 inches and a depth of 18 inches is standard. Use a grade level or laser level to verify that the bottom of the trench is continuously sloped downhill toward the discharge outlet. Smooth the bottom of the trench, removing any large rocks or roots that could puncture the filter fabric or damage the pipe.

Step 3: Lining the Trench with Geotextile Fabric

Spread non-woven geotextile filter fabric (specifically rated for drainage, 4oz or 6oz weight) along the trench. Ensure the fabric lies flat against the bottom and sides of the trench, with the excess fabric draped over the sides of the trench. Non-woven fabric is essential; do not use woven weed-barrier fabric, as its water permeability is too low, which will cause water to pool on the outside of the trench rather than enter the drain.

Step 4: Installing the Gravel Bed and Perforated Pipe

Shovel a 2 to 3-inch layer of clean, washed 3/4-inch to 1.5-inch crushed stone over the fabric at the bottom of the trench. This layer acts as a bedding base. Lay the perforated drain pipe on top of this gravel layer. The perforations (holes) in the pipe must face downward. Water enters the trench from the sides and bottom, rises into the gravel voids, and enters the pipe from below, where it is carried away. Laying the holes facing upward allows dirt to fall into the pipe, leading to clogging.

Step 5: Backfilling and Wrapping the Envelope

Shovel clean washed stone over and around the pipe, filling the trench to within 3 to 4 inches of the surface. Fold the draped flaps of the geotextile fabric over the top of the gravel, overlapping them by at least 12 inches. This fabric envelope completely isolates the gravel and pipe from the surrounding topsoil, ensuring that soil particles cannot wash down and clog the system.

Step 6: Final Backfill and Landscaping

Cover the fabric wrap with 3 to 4 inches of native topsoil. Lightly compact the soil. Re-lay sod, spread grass seed, or cover the area with decorative river rock if you prefer an exposed dry creek bed look. Ensure the pipe's discharge point is capped with a rodent guard or pop-up emitter to prevent small animals from nesting inside and blocking the drain.

Frequently Asked Questions

Why must the holes in a perforated French drain pipe face downward?

Perforations must face downward because groundwater rises from the bottom of the trench up into the gravel bed. When the water level reaches the bottom of the pipe, it enters the holes and is drained away. Facing the holes upward would require the entire trench to fill with water before the pipe could transport it, and it would also allow sediment and dirt to fall into the pipe, causing blocks.

What is the difference between corrugated pipe and rigid PVC pipe for drainage?

Corrugated polyethylene pipe is flexible, cheap, and easy to snake around obstacles, but its ridged interior slows down water flow and traps debris, making it harder to clean with a drain snake. Rigid PVC pipe (SDR-35 or Schedule 40 perforated) has a smooth interior that maximizes water flow velocity and allows for easy cleaning, making it the preferred choice for professional installations despite higher material costs.

Can I wrap the perforated pipe itself in fabric socks instead of lining the trench?

Wrapping the pipe in a fabric sleeve (drain sock) is not recommended as a standalone solution in soils high in clay or silt. The silt will quickly blind (clog) the tight fabric wrap right at the pipe surface, preventing water from entering. Lining the entire trench with geotextile fabric provides a significantly larger surface area for water filtration, preventing clogging and ensuring long-term system function.

How do I maintain a French drain, and what is its typical lifespan?

A properly installed French drain wrapped in quality non-woven geotextile fabric can last 30 to 50 years with virtually no maintenance. To ensure longevity, install inline cleanout ports (Y-sweep pipes) at the beginning and midpoints of the drain run. This allows you to flush the system with a garden hose or snake it if tree roots or sediment build up.

Where should a French drain discharge its water?

A French drain must discharge water to a safe, legal outlet. Safe options include a dry well located on your property, a sloped exit point where the pipe daylighted on a hillside, a rain garden, or a connection to a municipal storm drain (where local codes allow). Never discharge a French drain onto a neighbor's property, directly onto a public roadway, or close to a septic field.

All calculations are estimates. Always consult a professional contractor for accurate quotes. See our Disclaimer for details.