French Drain Calculator

A French drain is a gravel-filled trench, usually with a perforated pipe at the bottom, that intercepts subsurface water and conveys it to a lower outlet. It works by offering a path of far higher hydraulic conductivity than the surrounding ground: clean 3/4 inch angular stone has roughly 30 to 40 percent void space and transmits water orders of magnitude faster than the silty clay around it, so soil water migrates toward the trench and then travels in the pipe. Perforations face downward so the pipe fills from the trench invert upward rather than catching only what percolates onto its crown. Materials scale directly with trench geometry. Volume in cubic feet is length times width times depth expressed in feet; divide by 27 for cubic yards, then multiply by roughly 1.35 to 1.5 tons per cubic yard for washed stone or river rock. Deduct the pipe, since 4 inch pipe displaces about 0.087 cubic feet per linear foot. Typical residential dimensions are 12 inches wide by 18 to 24 inches deep for yard drainage, and down to footing depth for a foundation drain. Slope should be at least 1 percent, about 1 inch of fall per 8 feet, toward a daylight outlet or dry well, and it must be verified with a level rather than assumed from the ground surface. The dominant failure mode is siltation: without a non-woven geotextile wrapping the stone, fines migrate into the voids and a drain in clay soil can clog within a few years.

The name comes from Henry Flagg French, a lawyer, farmer, and later Assistant US Treasury Secretary from Concord, Massachusetts, whose 1859 book Farm Drainage described trenches filled with stone and pipe for draining agricultural fields. French drew on English tile-drainage practice, which had been transformed shortly before by cheap machine-extruded clay pipe, and popularized the technique among American farmers. The system was agricultural infrastructure long before it became a landscaping product sold by the linear foot. The governing physics arrived at almost the same moment. Henry Darcy, working on the municipal water supply of Dijon, published in 1856 the relationship now known as Darcy law, which states that flow through a porous medium is proportional to the hydraulic gradient and to the permeability of the material. That is the formal basis for choosing open-graded stone over sand or native backfill. Twentieth century changes were mostly in materials rather than in concept: corrugated perforated high-density polyethylene pipe, standardized under specifications such as ASTM F405, displaced clay tile and rigid pipe from the 1960s onward because it ships in coils and installs with few joints, while non-woven geotextile filter fabrics developed for civil engineering works in the same period finally addressed the siltation problem that had limited the service life of earlier stone drains.