FrenchDrainCalc Tip

French drain fabric calculator

Non-woven filter fabric (geotextile) lines the trench and folds over the top of the gravel to keep soil out. To wrap it fully, the strip must cover both walls and the bottom with about 12 in of overlap: 2 × depth + 2 × width + 12 in. FrenchDrainCalc works out that width, the total area, and the number of rolls. A 12 in wide, 18 in deep trench needs a 6 ft wide strip; with a 1% slope over 50 ft the outlet end is deeper and the strip grows to 6.5 ft.

Trench, pipe and fabric

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    If the strip is wider than your roll, overlap two strips along the bottom by at least 12 in.

    How the French drain calculator works

    FrenchDrainCalc treats the drain as a rectangular trench. With a slope, the bottom drops by length × slope while the top stays at ground level, so the average depth is the high-end depth plus half the fall. Gravel = trench volume − pipe volume (π × radius² × length), converted to cubic yards and to tons at 1.4 tons per cubic yard of washed drain rock, or to 0.5 cu ft bags. Filter fabric that wraps the whole trench needs a strip as wide as both walls plus the bottom and a 12 in overlap on top: 2 × depth + 2 × width + 12 in. Pipe is counted in 10 ft sticks or 100 ft coils, and the excavated soil swells about 25% once dug.

    Gravel for 10 ft of level trench, 4 in pipe

    TrenchGravelTonsFabric width
    6 × 12 in4.13 cu ft (0.153 cu yd)0.2144 ft
    8 × 18 in9.13 cu ft (0.338 cu yd)0.4735.33 ft
    12 × 18 in14.13 cu ft (0.523 cu yd)0.7336 ft
    12 × 24 in19.13 cu ft (0.708 cu yd)0.9927 ft
    18 × 24 in29.13 cu ft (1.079 cu yd)1.518 ft
    24 × 36 in59.13 cu ft (2.19 cu yd)3.06611 ft

    Gravel density varies with stone size and moisture (about 1.3 to 1.5 tons per cubic yard), so ask your supplier. Call 811 before digging to have buried utilities marked, keep the outlet lower than every point of the drain, and check local rules before sending water toward a neighbour or the street. Prices are your own estimates; results are for planning.

    Frequently asked questions

    How wide should French drain fabric be?

    Twice the depth plus twice the width plus 12 in of overlap. A 12 × 24 in trench needs a 7 ft (84 in) strip.

    Should I use landscape fabric for a French drain?

    Use non-woven geotextile drainage fabric; woven landscape fabric can clog with fine soil.

    Do I need a sock on the pipe too?

    Not if the trench is wrapped in fabric; a sock on the pipe as well can clog in silty soil.

    Is FrenchDrainCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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