FrenchDrainCalc Tip

French drain calculator

Enter the length of the drain, the trench width and depth and the pipe size, and FrenchDrainCalc works out everything you need to buy: drain gravel (trench volume minus the pipe), perforated pipe in 10 ft sticks or 100 ft coils, filter fabric to wrap the trench, and how much soil you will dig out. The slope deepens the trench toward the outlet. The example is 50 ft of 12 in wide, 18 in deep trench with 4 in pipe at a 1% slope: 3.08 cu yd of gravel.

Trench, pipe and fabric

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    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 much gravel do I need for a French drain?

    Trench length × width × depth, minus the pipe, in cubic feet ÷ 27 for cubic yards. A 50 ft, 12 × 18 in level trench with 4 in pipe needs about 2.6 cu yd (3.7 tons).

    How wide and deep should a French drain be?

    Commonly 6–12 in wide and 18–24 in deep for yard drainage, deeper for foundation drains.

    What size gravel for a French drain?

    Washed, clean angular stone around 3/4 to 1 1/2 in, so water flows freely and fines do not clog the pipe.

    Is FrenchDrainCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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