French Drain Materials Calculator

Straight trench length (ft). Measure the length of the straight exterior drainage trench. The pipe quantity follows this same run and may consist of several perforated pipe sections joined end-to-end. Calculate separate branches or changes in trench size separately; fittings, connections and discharge arrangements require additional materials.

Trench width (in). Trench width is the distance across the excavated trench, from one side to the other. This is the space occupied by the drain pipe and surrounding gravel. Use the actual planned width, not the pipe diameter. A wider trench requires more gravel and fabric; confirm that it accommodates the pipe and drainage material.

Total gravel-filled trench depth (in). Measure the full vertical depth of the gravel-filled trench section. It includes the gravel below the pipe, the pipe itself and the gravel above it. The calculator fills this entire section around the pipe with gravel; it does not reserve an upper layer for soil or paving.

Pipe outside diameter (in). Enter the pipe’s actual outside diameter from the product information or a direct measurement. Nominal pipe size may differ from outside diameter. The outside dimension determines both whether the pipe fits and how much gravel volume it displaces.

Minimum gravel below pipe (in). Enter the minimum gravel thickness required between the trench bottom and the underside of the pipe. Obtain this from the drainage detail appropriate to the project. The diagram places the pipe at this bedding height; it does not choose a drainage specification.

Minimum gravel above pipe (in). Enter the minimum gravel thickness required above the pipe. Together with bedding and outside pipe diameter, it must fit within the total trench depth. The diagram reports the actual remaining cover, which may be greater than this minimum.

Fabric top overlap (in). Enter the overlap required where the fabric closes over the gravel at the top. This length is added to the fabric needed to surround the full trench section. Use the selected fabric and installation detail; the calculator does not prescribe an overlap.

Fabric roll width (ft). Enter the width of the fabric roll you plan to purchase. It must be wide enough to wrap the trench cross-section and provide the entered top overlap in one width. The calculator does not design a wrap assembled from side-by-side narrow strips.

Fabric roll length (ft). Enter the length supplied on one fabric roll. The required net length includes the trench run and fabric to fold across both ends. When more than one roll is needed, the entered end-splice overlap reduces the useful added length of each further roll.

Fabric end splice overlap (in). Enter the length lost where one fabric roll overlaps the end of the next along the trench. This is different from the overlap closing the top of the wrap. Roll count includes this loss; the net wrap area does not include the extra material at splices.

Gravel allowance (%). Enter an additional percentage for gravel handling loss, uneven excavation or other project conditions. This allowance applies only to gravel. It does not lengthen the pipe, enlarge the fabric wrap or add fabric splice overlaps.

Gravel volume fills the entered trench section after subtracting the pipe’s outside volume, then adds the gravel allowance. Pipe length follows the trench run. Required fabric width wraps the complete section and adds the top overlap; net fabric length includes folds across both trench ends. Net wrap area excludes splice material, while roll count accounts for the entered end-splice losses. Use calculated minimums when checking roll size, rather than rounding a fractional reference down. This estimates materials for an exterior gravel-filled drain, not an entire interior basement waterproofing system.

Net gravel = length × (trench width × gravel depth − π × outside pipe diameter² ÷ 4), in feet. Apply gravel allowance and divide by 27 for yards. Fabric width = 2 × (width + depth) + top overlap. Fabric length = trench length + 2 × depth for end folds. If fabric is needed, rolls = max(1, ceil((fabric length − splice overlap) ÷ (roll length − splice overlap))). All lengths are in feet.

Worked example: A 30 ft trench, 12 in wide × 18 in total gravel-filled trench depth, with 4 in outside-diameter pipe and 3 in bedding/cover needs about 1.5697 cu yd of gravel before allowance. A 6 in fabric overlap requires a 5.5 ft-wide wrap and 33 ft length, or one 6 × 100 ft roll.

Material estimate only, not drainage engineering. Use a straight trench with a consistent cross-section and one drain line surrounded by gravel. Several connected pipe sections can form that line. Pipe fittings, slope, outlets, soil suitability, fabric selection and installation details are excluded. Enter end splice overlap separately; roll count accounts for the lost length at each splice. Extra roll width is included in purchased rolls.