French Drain Calculator

1. Enter the drain length in feet. Measure along the path where the trench will run.
2. Enter the trench width in inches. A standard residential French drain is 12 inches wide.
3. Enter the trench depth in inches. Most drains are 12 to 24 inches deep.
4. Enter the pipe diameter in inches. Standard residential pipe is 4 inches.
5. Enter the gravel price per ton in dollars to get a cost estimate.
6. Read your gravel volume, gravel weight, fabric area, pipe length and estimated cost.

A French drain is a gravel-filled trench with a perforated pipe that redirects surface water and groundwater away from a building foundation, retaining wall or low-lying area. Water seeps through the gravel, enters the pipe through small holes and flows to a safe outlet such as a dry well, storm drain or lower part of the yard.

The trench is lined with landscape fabric to keep soil from clogging the gravel and pipe. The gravel provides a porous channel for water to move through, and the pipe carries the collected water along the length of the drain to its exit point.

French drains are one of the most common residential drainage solutions. They work well along foundations, behind retaining walls, at the base of slopes and across soggy lawn areas.

The calculator uses basic volume geometry to estimate the materials for a rectangular trench.

1. The trench volume is calculated as a simple rectangular box in cubic feet.
2. The pipe cross-section is treated as a circle and its volume is subtracted from the trench.
3. The remaining space is how much gravel you need, converted to cubic yards and tons.
4. The fabric lines the bottom and both sides, so its width is twice the depth plus the trench width.

Take a 50-foot French drain, 12 inches wide and 18 inches deep, with a 4-inch pipe and gravel at $50 per ton.

Trench volume: 50 x (12/12) x (18/12) = 50 x 1 x 1.5 = 75 cu ft.

Pipe volume: pi x (4/24)^2 x 50 = pi x 0.02778 x 50 = 4.36 cu ft.

Gravel volume: 75 - 4.36 = 70.64 cu ft = 70.64 / 27 = 2.62 cu yd.

Gravel weight: 70.64 x 100 / 2000 = 3.53 tons.

Landscape fabric: 50 x (2 x 1.5 + 1) = 50 x 4 = 200 sq ft.

Pipe: 50 ft of 4-inch perforated pipe.

Gravel cost: 3.53 x $50 = $176.59.

The best gravel for a French drain is 3/4-inch washed or crushed stone. This size is large enough to let water flow freely but small enough to fill around the pipe without large gaps.

Common Gravel Options

Avoid pea gravel, sand or anything with fine particles. Fines fill the gaps between stones, slow drainage and clog the fabric. This calculator uses a density of 100 lb per cubic foot, which is typical for 3/4-inch washed gravel.

Getting the Slope Right

A French drain must slope downhill so water flows to the outlet. The recommended slope is 1% or about 1 inch of drop per 8 feet of drain length. A 50-foot drain should drop at least 6 inches from start to end.

Where to Place a French Drain

• Along a foundation wall to intercept water before it reaches the basement.
• Behind a retaining wall to relieve hydrostatic pressure.
• Across a low area in the yard that collects standing water.
• At the base of a slope to catch runoff before it saturates the lawn.

Pipe Placement

Lay the perforated pipe with the holes facing down. Water rises into the pipe from below. Set the pipe on a 2-3 inch bed of gravel, wrap fabric around the pipe, then fill the trench with gravel to within a few inches of the surface.

Several factors change how much gravel, pipe and fabric a project requires beyond the basic trench dimensions.

Trench Depth and Width

Deeper and wider trenches need more gravel per foot. A 24-inch deep trench uses 33% more gravel than an 18-inch trench of the same length and width.

Pipe Diameter

A larger pipe displaces more gravel, so you buy slightly less stone. However, 6-inch pipe is heavier and more expensive than 4-inch, so the savings on gravel are usually offset.

Gravel Type and Density

This calculator uses 100 lb per cubic foot, typical for 3/4-inch washed stone. River rock is lighter (about 90 lb/cu ft) and lava rock lighter still. Adjust the price per ton if you switch types.

Settling and Waste

Gravel settles 5-10% after the trench is backfilled. Most suppliers recommend ordering at least 5% extra to top off after settling.

Fabric Overlap

Landscape fabric rolls come in standard widths. You may need extra fabric for overlapping seams and wrapping the pipe, which this calculator does not include.

Material costs vary by region and supplier. The table below gives typical 2025 price ranges for the main components of a residential French drain.

DIY vs Professional Installation

DIY materials for a 50-foot drain typically run $200-$400. Professional installation adds labor and equipment, bringing the total to $1,250-$3,750 ($25-$75 per linear foot). The biggest labor cost is digging the trench, which a trenching machine speeds up considerably.

Water Pooling Near the Foundation

Standing water within 3 feet of a foundation wall is the most common reason to install a French drain. The drain intercepts water before it can seep into the basement or crawl space.

Soggy Lawn Areas

Low spots that stay wet for days after rain benefit from a shallow French drain that redirects water to a drier part of the yard or a dry well.

Behind a Retaining Wall

Hydrostatic pressure behind a retaining wall can cause it to lean or fail. A French drain at the base of the wall relieves that pressure. See the retaining wall calculator for block and backfill estimates.

Downhill Slopes Draining Toward a Structure

A French drain across the slope catches runoff before it reaches a patio, garage or walkway.

Before Landscaping or Hardscaping

It is much easier to install a drain before laying sod, pavers or a deck. Plan drainage early to avoid tearing up finished work later.

1. No Slope or Wrong Slope

Without a consistent downhill slope the water sits in the pipe instead of draining. Check the grade before backfilling.

2. Using the Wrong Gravel

Sand, pea gravel or dirty stone with fines clogs the fabric and pipe. Use clean, washed 3/4-inch stone.

3. Skipping the Landscape Fabric

Without fabric, soil migrates into the gravel and blocks water flow within a few years.

4. Connecting to a Sealed Pipe

The outlet pipe should be solid, not perforated, so collected water exits without leaking back into the ground.

5. Digging Too Shallow

A drain that is only 6-8 inches deep may not intercept subsurface water. Most residential drains need 12-24 inches of depth.

6. No Outlet

Every French drain needs a place for water to go. Without an outlet, the trench fills up and stops working.

The math is exact for a rectangular trench with a round pipe, but real-world conditions introduce small variations.

Most residential French drains are 12 to 24 inches deep. The drain must be deep enough to intercept subsurface water and slope to an outlet. Foundation drains are often 18 to 24 inches; lawn drains can be as shallow as 12 inches.

A slope of about 1% is standard, which is roughly 1 inch of drop per 8 feet of length. This keeps water moving through the pipe without running too fast and eroding the gravel bed.

Use 3/4-inch washed or crushed stone. It is large enough to let water flow freely and small enough to support the pipe. Avoid pea gravel, sand or stone with fine particles because they clog the fabric over time.

A 4-inch perforated pipe handles most residential French drains. For longer drains or heavy water volume, upgrade to a 6-inch pipe. Use solid pipe for the outlet section where water exits the system.

A properly installed French drain with landscape fabric typically lasts 20 to 30 years. Without fabric, silt clogs the gravel in 5 to 10 years and the drain loses effectiveness.

Material cost depends on length and local gravel prices. A 50-foot drain typically costs $150-$300 in materials (gravel, pipe, fabric, fittings). Professional installation runs $25-$75 per linear foot, including labor and equipment.

Yes. Landscape fabric keeps soil from migrating into the gravel and clogging the drain. Wrap it around the entire gravel fill, overlapping seams by at least 6 inches.

A trench filled only with gravel can move some water, but a perforated pipe greatly increases capacity and lifespan. The pipe collects water from the gravel and moves it along the drain much faster than gravel alone.

The outlet should discharge to a lower area of the yard, a dry well, a storm drain or a rain garden. Water must be directed away from the foundation and neighboring properties. Check local codes for outlet rules.

For a 12-inch wide by 18-inch deep trench with a 4-inch pipe, you need about 1.41 cubic feet of gravel per foot of drain, which is roughly 0.071 tons per foot at 100 lb per cubic foot density.

• University of Minnesota Extension -- Managing water on your property (French drain design principles, trench dimensions and gravel selection for residential drainage).
• Aggregate density reference -- Gravel and crushed stone (Typical bulk densities for common aggregates; 3/4-inch washed gravel approximately 95-105 lb per cubic foot).
• National Association of Home Builders -- Drainage solutions (Residential drainage best practices, including French drain slope and outlet guidance).