FrenchDrainCalc Tip

French drain pipe calculator

Perforated pipe at the bottom of the trench carries water away much faster than gravel alone. FrenchDrainCalc counts the 10 ft rigid sticks or 100 ft corrugated coils for your drain length and takes the pipe volume out of the gravel. A 4 in pipe takes up about 0.087 cu ft per foot; a 6 in pipe about 0.2.

Trench, pipe and fabric

    Saved you some time? Leave a tip, entirely optional.

    Lay rigid perforated pipe with the holes facing down, and add a cleanout at the high end if you can.

    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

    What size pipe for a French drain?

    4 in perforated pipe is standard for yards and foundations; 6 in is used for long runs or heavy water.

    Should French drain pipe holes face up or down?

    Down, for rigid pipe with two rows of holes, so water rises into the pipe and sediment does not settle in it.

    Rigid or corrugated pipe?

    Rigid PVC is easier to keep at a steady slope and to clean; corrugated coil is cheaper and bends around corners.

    Is FrenchDrainCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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