FrenchDrainCalc Tip

French drain gravel calculator

Most of a French drain is stone, so gravel is the biggest material cost. FrenchDrainCalc multiplies the trench length, width and average depth and subtracts the space the pipe takes up. Drain rock weighs about 1.4 tons per cubic yard; bagged gravel usually comes in 0.5 cu ft bags, which gets expensive quickly: the 50 ft example needs 167 bags, so bulk delivery is usually cheaper above a cubic yard or so.

Trench, pipe and fabric

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    Choose "None" for the pipe to size a gravel-only drain or a dry well trench.

    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 many tons of gravel for a 100 ft French drain?

    For a 12 × 18 in trench with 4 in pipe at a 1% slope, about 9.9 tons (7.1 cu yd); level, about 7.3 tons.

    How many bags of gravel for a French drain?

    Gravel in cubic feet ÷ 0.5 for half-cubic-foot bags; a 10 ft, 12 × 18 in trench takes about 29 bags.

    How much does a cubic yard of gravel weigh?

    About 1.4 tons (2,800 lb) for washed drain rock; it varies from about 1.3 to 1.5.

    Is FrenchDrainCalc free?

    Yes, with no sign-up or ads.

    Are my numbers saved?

    Yes, in this browser only.

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