Quantities first, because gravel is bought by the ton and trenches are dug by the yard
A French drain is a trench, a pipe and a great deal of stone, and its cost follows the quantities: the trench volume sets the digging, the stone that remains after the pipe takes its space sets the gravel order, and the fabric that wraps it all is measured off the trench’s inside surface. This page works those quantities out from your trench dimensions, then prices them at your rates: excavation per cubic yard, gravel per ton delivered (at your stone’s density, with your waste allowance), pipe per foot, fabric per square foot, labour per foot and spoil disposal, then fittings. It checks the slope over the run, applies any minimum charge openly, adds contingency once and tax once, and reports the total and the cost per foot of drain, with a downloadable estimate.
The trench
Pipe, gravel and fabric
Minimum, contingency, tax and range
Disclaimer: This calculator gives estimates for general information only. It is not financial, medical or other professional advice. Drainage work interacts with foundations, boundaries and local rules on where water may be discharged; the outlet must be legal as well as downhill, and neither is checked by an estimate.
What this calculator does
It treats a French drain as the quantity problem it is. The trench geometry produces three volumes that price three different things: the full trench volume is what gets dug and (where it leaves the site) disposed of; the trench minus the pipe’s own volume is the stone that stays in the ground, converted to tons at the entered density because stone is sold by weight; and the trench’s inside surface, width plus both walls plus an overlap allowance, is the fabric that lines it. The run’s fall is converted to a slope and set against the usual minimum guidance, because a drain at the wrong gradient is an expensive buried tube. Only once the quantities stand does the pricing begin, at rates the user enters for each, with the estimator discipline to close: minimum charge openly, contingency once, tax once, range, and the per-foot figure the whole job is usually quoted in.
How to use it
- The outlet decides the trench, not the other way round. Water needs somewhere lawful to go: daylight on a slope, a dry well, a storm connection where permitted. The fall field prices nothing, but a run that cannot achieve its gradient to a real outlet should not achieve a budget either; solve the outlet first.
- Stone quantity is trench minus pipe, and both terms matter. On the worked example the 4 inch pipe displaces 5.24 cubic feet of a 180 cubic foot trench; small, but the same arithmetic on a bigger pipe in a narrower trench moves the order by tons. Suppliers sell by the ton and trenches are dug by the yard, which is why the density field sits between them.
- Fabric is measured off the walls, not the plan. The lining wraps the trench’s inside: floor, both sides, and enough over the top to close. The overlap allowance field is the difference between the fabric that arrives and the fabric the geometry strictly needs, and skimping it is how linings gap at the crown where the soil is closest.
- Depth buys performance and costs twice. A deeper trench intercepts water moving lower in the ground and also means more digging, more stone and more spoil, all three priced here off the same depth figure. Set depth from where the water actually travels, not from habit: too shallow is a drain that misses its water, too deep is stone bought for nothing.
- Washed stone and fabric are the system, not upgrades. The prices assume drainage stone and a fabric wrap doing the filtering; substituting crusher run or skipping the lining changes the drain’s working life, not just its price. The fields price the components separately so substitutions show their saving honestly, against the limitation below.
How it is calculated
- Trench volume = length x width x depth, in cubic feet; excavation yards = cubic feet / 27. Pipe displacement = pi x (diameter in feet / 2) squared x length. Gravel volume = trench volume less pipe displacement. Gravel tons in place = gravel volume x density / 2,000. Gravel to order = tons in place x (1 + waste %).
- Fabric area = length x (width + 2 x depth + overlap in feet). Slope = fall (in feet) / length, as a percentage.
- Lines: excavation = yards x rate; gravel = tons to order x price per ton; pipe = length x cost per foot; fabric = area x rate per square foot; labour = length x rate per foot; spoil = excavation yards x disposal rate; fittings enter flat. Money lines are computed on unrounded quantities and rounded to cents; quantities print to two decimals.
- Subtotal = the sum of the line amounts. Minimum charge: if a minimum is entered and exceeds the subtotal, the minimum becomes the base and the results state that the minimum charge applied. Contingency applies once to the base; tax once to base plus contingency. Total = base + contingency + tax. Per foot = total / trench length. Range = total less and plus your percentage.
Worked examples
A 60 foot drain, fully priced. Illustrative rates: a trench 60 feet by 1.5 wide by 2 deep is 180 cubic feet, 6.67 cubic yards to dig. The 4 inch pipe displaces 5.24 cubic feet, leaving 174.76 cubic feet of stone: 6.47 cubic yards, 9.18 tons in place at 105 pounds per cubic foot, 10.09 tons to order with 10% extra. Fabric: 1.5 + 4 + 1 foot of overlap is 6.5 feet of width, 390 square feet. Excavation at $38 a yard is $253.33; gravel at $48 a ton on the order quantity is $484.45; pipe at $1.85 a foot is $111.00; fabric at $0.18 is $70.20; labour at $14 a foot is $840.00; spoil disposal at $22 a yard is $146.67; fittings $65. Subtotal $1,970.65; contingency at 10% adds $197.07 for a total of $2,167.72, or $36.13 per foot. A fall of 12 inches over the run is a 1.67% slope, comfortably past the usual 1% guidance.
Why the stone order surprises people. The trench a tape measure sees is 6.67 cubic yards; the stone invoice is 10.09 tons. Between the two numbers sit a density (105 pounds in every cubic foot) and a unit change (2,000 pounds to the ton), which together turn modest trenches into serious tonnage. Orders placed in yards against deliveries in tons are the standard French drain mishap, and it is prevented by exactly the conversion printed in these results.
A fittings-only job meets the floor. A $40 basin and outlet repair against a $250 minimum charge for attended drainage work: the estimate builds on the minimum and totals $250.00, stating that the minimum charge applied.
Reading your results
Check the slope verdict before the total: a drain that cannot fall is a drain that cannot work, and no accuracy in the stone order repairs it. Then read the labour share: on the worked example labour is $840 of a $1,970.65 subtotal, the largest single line, which is typical of shallow trenches and is why machine digging (priced in the excavation rate when the contractor digs by machine) changes installed prices more than any material negotiation. Finally, sanity-check the tons against the trench: any quote whose stone quantity cannot be reconciled to trench-minus-pipe at a stated density is a quote with a hidden assumption in it.
Limitations
- Soil and groundwater are not in the fields. Trenches in collapsing sand, high water tables or rock dig slower, shore differently and sometimes pump continuously. The excavation rate must be the rate for the ground the trench is actually in; the page converts geometry faithfully and knows nothing of geology.
- Slope guidance is general, not design. The roughly 1% figure is the common minimum for a drain run; steeper falls move water faster and can matter with fine soils, and local rules or the outlet’s level may dictate the profile. A drain protecting a foundation deserves design, not guidance.
- Discharge law is local. Routing water to a neighbour, a watercourse or a storm system is regulated in many places. This page prices the trench; where its water may go is a question for the local rules, asked before the digging.
- One prism stands in for real trenches. Width and depth are treated as constant along the run. Stepped or varying trenches are priced honestly by running the page per section and adding the estimates.
Frequently asked questions
How much gravel does a French drain need?
The trench’s volume minus the pipe’s volume, converted at the stone’s density: on the worked example, a 180 cubic foot trench with a 4 inch pipe leaves 174.76 cubic feet of stone, which at 105 pounds per cubic foot is 9.18 tons in place and 10.09 tons to order with a 10% allowance. The two conversions that catch people are the pipe displacement (small pipes, small effect; big pipes in narrow trenches, real effect) and yards to tons, which this page prints rather than assumes.
What slope does a French drain need?
A continuous fall from inlet to outlet, commonly at least about 1%, roughly an inch in eight feet, so water moves by gravity the whole way and sediment does not settle in a flat belly. This page converts your total fall over the run into that percentage: 12 inches over 60 feet is 1.67%. More fall is generally workable; less, or a run with a low spot in the middle, ponds inside the pipe where no gradient can rescue it.
Why line the trench with fabric?
To keep soil out of the stone. An unlined drain slowly fills its gravel voids with migrating fines until the trench is a buried bag of soil that water passes around rather than through. The fabric wraps the stone (floor, walls and a closed overlap over the top), and its area is measured off those surfaces here, which is why a 60 foot trench needs 390 square feet of it rather than the 90 a plan view suggests.
Can the excavated soil go back on top?
Yes, the crown of the trench is commonly backfilled and turfed or surfaced once the fabric is closed over the stone, which is why spoil disposal here is charged on the excavation volume only when spoil actually leaves. Where spoil stays, leave the disposal rate blank and the line disappears; where it goes, it goes at loose volume in practice, and a contractor’s per-yard disposal rate will already carry that understanding.
Related calculators
Method: trench geometry converted to excavation yards, stone tons at the entered density and fabric area with overlap, priced line by line at user-entered rates; minimum charge applied to the base, contingency once and tax once, exactly as defined on this page. Last reviewed: 13 October 2026.