FrenchDrainCalc Tip

French drain slope calculator

A French drain needs a steady fall to its outlet; 1% (about 1/8 in per foot, or 1 in every 8 ft) is a common minimum, and more is better. FrenchDrainCalc turns length and slope into fall and the outlet depth. This page starts with a 100 ft drain: at 1% it falls 12 in, so an 18 in deep trench at the top end is 30 in deep at the outlet, and the extra depth adds gravel and digging.

Trench, pipe and fabric

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

    If the ground already slopes toward the outlet, you can keep the trench at a constant depth and set the slope here to 0.

    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 slope does a French drain need?

    At least 1% (1 ft per 100 ft, about 1/8 in per foot). Some guides suggest 2% where possible.

    What is the fall over 50 ft at 1%?

    6 inches: 50 ft × 12 in × 1% = 6 in.

    Can a French drain be level?

    A level drain will hold water and works poorly; give it a slope to a free outlet, a dry well or a sump.

    Is FrenchDrainCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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