French Drain Calculator
Planning a French drain to manage water around your foundation or yard? Enter the trench dimensions and gravel type to calculate the exact materials needed including gravel tonnage, pipe length, and landscape fabric.
Estimate only — not professional advice. Always verify results independently before purchasing materials or beginning work. Terms of Use
What is French Drain?
A French drain calculator determines the quantity of materials needed to construct a subsurface drainage system — specifically the gravel fill volume (in cubic yards and tons), perforated pipe length, and filter fabric area. A French drain is a trench filled with gravel surrounding a perforated pipe that collects and redirects groundwater away from foundations, retaining walls, or low-lying yard areas. The standard French drain trench is 12 inches wide and 18 inches deep, though dimensions vary based on drainage requirements. The trench is lined with filter fabric (geotextile) to prevent soil from migrating into the gravel and clogging the system. A 4-inch perforated pipe (holes facing down in most installations) sits on a bed of gravel at the bottom of the trench, and additional gravel fills the trench around and above the pipe. The filter fabric wraps over the top of the gravel before being covered with topsoil or sod. The gravel volume calculation subtracts the volume occupied by the 4-inch diameter pipe, which displaces approximately 0.087 cubic feet per linear foot of trench. While this displacement is small relative to the total trench volume, it adds up over long runs and prevents over-ordering. The gravel must be clean, washed stone — typically 3/4-inch to 1-1/2-inch crushed stone or 3/8-inch pea gravel. Never use limestone screenings, sand, or unwashed material that contains fines — these clog the pipe perforations and defeat the drainage purpose. Pea gravel weighs approximately 1.4 tons per cubic yard and provides excellent drainage with its rounded shape allowing water to flow freely between particles. Crushed stone (angular) weighs approximately 1.5 tons per cubic yard and provides slightly better structural support for the trench walls but marginally slower drainage. Either material works well for French drains when properly sized (3/4-inch to 1-1/2-inch). Landscape fabric (geotextile) is calculated to line the sides and bottom of the trench with enough overlap at the top. The formula calculates fabric as a U-shape lining — covering both walls and the bottom — then multiplied by length. Non-woven geotextile rated at 4 oz per square yard or heavier is recommended for French drains. Lighter fabrics deteriorate within 5 to 10 years underground. Proper French drain installation requires a minimum slope of 1 percent (1/8 inch per foot) toward the discharge point. The drain must outlet to daylight (a slope, ditch, or storm drain) or a dry well. Dead-end French drains with no outlet eventually fill with water and become ineffective. The pipe should be rigid Schedule 40 PVC with pre-drilled holes or corrugated HDPE drain pipe wrapped in a filter sock — avoid the thin single-wall corrugated pipe that collapses under backfill weight.
How to Calculate
- Measure the total length of the French drain run in feet (following the drainage path)
- Set the trench width (12 inches is standard, 6 inches minimum for tight spaces)
- Set the trench depth (18 inches standard, deeper for foundation drainage)
- Select your gravel type (pea gravel for maximum flow, crushed stone for stability)
- Review the gravel volume in cubic yards and tons for ordering from a landscape supply yard
- Note the landscape fabric square footage to purchase the correct roll length
- Pipe length equals drain length — order 4-inch perforated pipe in this quantity
Formula
Trench Volume (cu ft) = Drain Length x (Width / 12) x (Depth / 12) Pipe Volume (cu ft) = pi x (2/12)^2 x Drain Length (for 4-inch diameter pipe) Gravel Volume (cu ft) = Trench Volume - Pipe Volume Gravel (cu yd) = Gravel Volume / 27 Gravel (tons) = Gravel (cu yd) x Density (1.4 for pea gravel, 1.5 for crushed stone) Landscape Fabric (sq ft) = 2 x ((Width + Depth) / 12) x Drain Length Pipe Length = Drain Length
Example Calculation
50 ft drain, 12 in wide, 18 in deep, pea gravel: Trench Volume = 50 x (12/12) x (18/12) = 50 x 1 x 1.5 = 75 cu ft Pipe Volume = pi x (2/12)^2 x 50 = 3.14159 x 0.02778 x 50 = 4.36 cu ft Gravel Volume = 75 - 4.36 = 70.64 cu ft Gravel (cu yd) = 70.64 / 27 = 2.62 cu yd Gravel (tons) = 2.62 x 1.4 = 3.67 tons Landscape Fabric = 2 x ((12 + 18) / 12) x 50 = 2 x 2.5 x 50 = 250 sq ft Pipe Length = 50 ft
Frequently Asked Questions
How deep should a French drain be?
Standard French drains are 18 to 24 inches deep. For foundation waterproofing, the drain should extend to the bottom of the footing — typically 24 to 36 inches deep. For yard drainage to intercept surface water, 12 to 18 inches is often sufficient. The drain must be deep enough that the perforated pipe sits below the water table you are trying to lower.
What size gravel is best for a French drain?
Use 3/4-inch to 1-1/2-inch clean washed stone. This size provides the optimal balance between drainage capacity (large voids for water flow) and pipe support. Avoid pea gravel smaller than 3/8 inch (too fine, may enter pipe perforations) and stone larger than 2 inches (poor pipe bedding). The stone must be washed — fines and dust will clog the system within a few years.
Do I need landscape fabric for a French drain?
Yes. Geotextile fabric is essential to prevent soil from migrating into the gravel fill and clogging the drainage voids. Without fabric, silt gradually fills the spaces between gravel particles, and the drain becomes a solid mass that cannot conduct water — typically within 3 to 10 years depending on soil type. Use non-woven geotextile rated at 4 oz/sq yd minimum.
What slope does a French drain need?
A French drain requires a minimum slope of 1 percent (1/8 inch per foot, or about 1 inch per 8 feet) toward the outlet. Steeper slopes move water faster and reduce the risk of sediment accumulation. A 2 percent slope (1/4 inch per foot) is ideal when the terrain allows. Always verify slope with a laser level during installation — eyeballing is not accurate enough over distances greater than 10 feet.
Where should a French drain discharge?
A French drain must discharge to a legal outlet: daylight on a downhill slope, a storm drainage ditch, a municipal storm sewer connection (with permit), or a dry well / infiltration pit at least 10 feet from any structure. Never discharge to a sanitary sewer, septic system, or neighboring property. If no gravity outlet exists, a sump pit with a pump may be required.