French Drain Calculator
A French drain is one of the most effective ways to redirect groundwater away from your foundation, yard, or other low-lying areas. This calculator determines the gravel, perforated pipe, and landscape fabric needed for your French drain based on trench dimensions, helping you order materials accurately.
A French drain consists of a trench filled with gravel surrounding a perforated pipe, all wrapped in landscape fabric to prevent soil from clogging the gravel. Water seeps through the gravel, enters the perforated pipe, and is carried away from the problem area by gravity.
Standard residential French drains use a 12-inch wide by 18-inch deep trench with a 4-inch perforated pipe. The pipe sits on 2-3 inches of gravel at the bottom, and the rest of the trench is filled with 3/4-inch washed drainage gravel. The trench should slope at least 1% (1 inch per 8 feet) toward the outlet.
Use washed, angular gravel (not round river rock) for best drainage. Landscape fabric is critical to prevent silt from migrating into the gravel and clogging the system. Wrap the entire trench before adding gravel, leaving enough overlap at the top to fold over the completed fill.