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French drain calculator
Enter your trench size and pipe and get the stone in cubic yards and tons, the pipe and filter fabric to buy, and how much fall the drain needs from end to end.
Clean stone with no fines, fabric that keeps the soil out, and a steady fall of at least 1% (some experts want 2% with corrugated pipe) to an outlet lower than the drain. Get those three right and the size is the easy part.
Washed #57 stone weighs about 1.35 to 1.4 tons per cubic yard, depending on the rock; put in your supplier’s number if you have it. If you’ll cap the trench with soil or sod, enter only the depth of stone, and use the average depth if the trench gets deeper toward the outlet. The overlap is where the wrap closes on top, and at any seam.
Check the highlighted box: sizes and stone weight greater than zero, an overlap from 0 to 24 in., and a trench wider and deeper than the pipe.
Stone fills the trench to the depth you entered. The pipe’s volume uses its nominal size, so the stone figure runs a little high, which covers spillage.
In central PA? We dig and install French drains with our own machines. See our drainage and site work.
How it works
Trench volume = length × width × depth, in feet. Stone = trench volume − pipe volume (π × radius² × length). Cubic feet ÷ 27 = cubic yards, × tons per cubic yard = tons.
Pipe = trench length + 5%, rounded up. Fabric wrap = 2 × depth + 2 × width + overlap, × length = square feet. Rolls are full-length strips laid side by side, each seam lapped by your overlap, plus the same overlap wherever one roll ends and the next begins.
At 1%, the trench bottom drops 1 ft per 100 ft, or 0.12 in. per foot; at 2%, 0.24 in. per foot. Length × the drop per foot = total fall from the high end to the outlet.
50 ft × 12 in. × 18 in.: 50 × 1 × 1.5 = 75 cu ft · 4 in. pipe: π × (2 ÷ 12)² × 50 = 4.4 cu ft
Stone: 75 − 4.4 = 70.6 cu ft = 2.62 cu yd × 1.4 = 3.66 tons · pipe: 50 × 1.05 = 52.5 → 53 ft
Fabric: 36 + 24 + 12 = 72 in. = 6 ft wide × 50 ft = 300 sq ft · fall at 1%: 50 × 0.12 = 6 in. · at 2%: 50 × 0.24 = 12 in.
Related: the gravel calculator and slope calculator, do I need a French drain or just regrading? and drainage laws by state, the rules on sending water toward a neighbor’s property.
Questions
How much gravel do I need for a French drain?
Multiply the trench length by its width and depth in feet, subtract the space the pipe takes up, and divide by 27 for cubic yards. A 50 foot trench 12 inches wide and 18 inches deep with a 4 inch pipe takes about 2.6 cubic yards of stone, or about 3.7 tons at 1.4 tons per cubic yard.
How much slope does a French drain need?
At least 1%, which is 1 foot of fall per 100 feet, or about 1/8 inch per foot. NDS, the drainage products maker, says in its French drain installation guide that a French drain should drop at least 1 percent, and notes that some experts call for a 2 percent minimum with corrugated pipe. A 50 foot drain drops 6 inches at 1% and 12 inches at 2%, and the water has to leave at or below that low end.
What kind of stone goes in a French drain?
Clean, washed stone with the fines screened out, so water moves through it. #57 stone, sold as 2B in Pennsylvania, is the common choice. Crusher run or 2A modified keeps its fines and packs tight, so it doesn’t belong in a drain.
How much does a yard of #57 stone weigh?
About 1.35 to 1.4 tons. Lorusso Corp. lists 3/4 inch washed crushed stone at 1.35 tons per cubic yard as loaded in the truck, and Select Sand & Gravel puts a cubic yard of clean crushed stone at 2,700 to 2,800 pounds. The type of rock changes it, so use your supplier’s number in the calculator.
How much filter fabric do I need for a French drain?
Enough to line the bottom and both sides and close over the top with an overlap: 2 × depth + 2 × width + overlap. A 12 by 18 inch trench with a 12 inch overlap needs a 6 foot wide wrap, or 300 square feet for 50 feet of drain. That is wider than a 3 or 4 foot roll, so either lay two or three strips side by side and lap the seams, or buy a roll wide enough to wrap the trench in one piece.
Sources: Lorusso Corporation conversion chart (3/4″ washed crushed stone 1.35 tons per cubic yard, as loaded; figures are guidelines); Select Sand & Gravel (clean crushed stone about 2,700–2,800 lb per cubic yard); York Building Products aggregate brochure (AASHTO #57 = PA 2B); NDS (Advanced Drainage Systems), French Drain Installation Guide (a French drain should drop at least 1 percent; some experts recommend a 2 percent minimum for corrugated pipe; 4 in. perforated pipe is likely adequate for most homes). 1 cu yd = 27 cu ft; 1% = 0.12 in. per ft; 2% = 0.24 in. per ft.
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