French Drain Calculator
Estimate gravel volume, pipe length, and trench dimensions for French drain installation. Free online French drain calculator with material breakdowns and visual charts.
About This Calculator
A French drain is an underground drainage system designed to redirect surface water and groundwater away from building foundations, basements, and low-lying areas. Named after Henry Flagg French, the system consists of a trench filled with gravel and a perforated pipe at the bottom that carries water to a designated outlet.
This calculator helps homeowners, contractors, and landscapers estimate the materials needed for a French drain installation. Enter your trench dimensions, pipe size, and gravel properties to get accurate volume, weight, and pipe length calculations. The formula accounts for trench volume, pipe displacement based on outside diameter, pipe slope for proper drainage, and gravel wastage for purchasing.
The trench volume is calculated as (Width / 12) × (Depth / 12) × Length (converting inches to feet). Pipe volume is subtracted using π × (Diameter / 24)² × Pipe Length. The pipe length accounts for slope using the Pythagorean theorem. Gravel density defaults to 105 lb/ft³ (1,680 kg/m³), which is standard for pea gravel and crushed stone.
Regional Notes
This calculator uses imperial units (feet, inches, pounds) which are standard for construction in the United States and UK. In metric regions, note that 1 cubic foot equals approximately 0.0283 cubic meters, and 1 pound equals 0.4536 kilograms. The recommended pipe slope of 0.125 inches per foot equals approximately 1 cm per meter.
Frequently Asked Questions
What is a French drain?
A French drain is an underground drainage system consisting of a trench filled with gravel and a perforated pipe at the bottom. It redirects surface water and groundwater away from foundations, basements, and low-lying areas to prevent flooding and water damage.
How does a French drain work?
Water seeps through the gravel into the perforated pipe at the bottom of the trench. The pipe is sloped downward so gravity carries the water to a designated outlet like a dry well, storm drain, or drainage ditch. The gravel prevents soil and debris from clogging the pipe.
How much gravel do I need per foot of French drain?
For a typical trench 10 inches wide and 12 inches deep with a 4-inch drain pipe, you need approximately 0.72 cubic feet of gravel per linear foot. For a 6-inch pipe in the same trench, you need about 0.6 cubic feet per foot. Our calculator accounts for pipe displacement and wastage.
What size pipe should I use for a French drain?
Standard French drain pipe sizes range from 4 inches to 6 inches in diameter. A 4-inch pipe is sufficient for most residential applications, while larger commercial or heavy-rainfall areas may require 6-inch or larger pipes. Schedule 40 PVC or corrugated perforated pipe are common choices.
What is the minimum slope for a French drain pipe?
For pipes up to 2.5 inches, use a minimum slope of 0.25 inches per foot. For 3 to 6-inch pipes, 0.125 inches per foot is recommended. For pipes 8 inches and larger, 0.0625 inches per foot is sufficient. Proper slope ensures water flows without pooling inside the pipe.
Do I need filter fabric for a French drain?
Yes, filter fabric (geotextile) is highly recommended. Lining the trench with filter fabric before adding gravel prevents silt, sand, and soil from clogging the gravel and pipe perforations. This extends the life of your French drain and reduces maintenance.
How deep should a French drain be?
French drains are typically 12 to 24 inches deep for residential applications. The depth depends on the water table level, soil type, and what you are protecting. The drain should be deep enough to intercept groundwater and slope toward the outlet.
Can I install a French drain without a pipe?
Yes, a pipeless French drain (also called a dry well or gravel-only trench) works for light drainage needs. It consists of a gravel-filled trench without a perforated pipe. However, pipe-based systems are more effective for heavy water flow and are recommended for most applications.