French Drain Calculator
Estimate the materials for a French drain before you rent the trencher. Enter the trench length, width, and depth and pick your pipe size to get the washed stone in cubic yards and tons, the square feet of filter fabric, the pipe run, and a stone cost.
Your project
Estimate
3.66tons
Washed stone needed · Most yards sell drainage stone by the ton
- Stone volume
- 2.62yd³
- Filter fabric
- 240ft²
- Perforated pipe
- 50ft
- Estimated stone cost
- $175.81
Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.
How it's calculated
No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.
- 1
Trench volume (ft³) = length × (trench width ÷ 12) × (trench depth ÷ 12) - 2
Pipe volume (ft³) = length × π × (pipe diameter ÷ 24)² - 3
Stone: cubic yards = (trench volume − pipe volume) ÷ 27, tons = cubic yards × 1.4 - 4
Fabric (ft²) = length × (trench width + 2 × trench depth) ÷ 12 × 1.20 for laps - 5
Cost = stone tons × price per ton
Worked example
A 50 ft French drain in a 12 in wide, 18 in deep trench with 4 in perforated pipe.
Inputs
- Drain length
- 50 ft
- Trench width
- 12 in
- Trench depth
- 18 in
- Pipe size
- 4
- Washed stone price
- 48 $/ton
Result
- Washed stone needed
- 3.66 tons
- Stone volume
- 2.62 yd³
- Filter fabric
- 240 ft²
- Perforated pipe
- 50 ft
- Estimated stone cost
- $175.81
Tools & materials for this project
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- 4 in perforated drain pipe The corrugated or rigid perforated pipe that carries the water out.
- Non-woven filter fabric Wraps the stone so soil fines cannot silt up the trench.
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Frequently asked questions
How much slope does a French drain need?
Aim for at least 1% fall, which is 1 inch of drop every 8 feet, or 1 foot over 100 feet. Over a 50 ft run that means the outlet end sits at least 6 inches lower than the head. Less than 1% and water sits in the pipe, silt drops out, and the drain slowly plugs. More slope is fine — 2% is better if the grade allows. Set the fall with a string line and a line level or a laser before you place any stone, and check it again after the pipe is bedded, because the stone under the pipe is easy to leave high.
Why washed angular stone instead of pea gravel?
Use washed, angular 3/4 inch drainage stone (often sold as No. 57 stone). Angular pieces bridge against each other and hold roughly 30–40% void space, so water moves freely and the trench walls do not collapse. Pea gravel is rounded and packs tighter, which cuts void space and lets the stone migrate and settle. The word "washed" matters just as much: unwashed crusher run carries fines that wash down and seal the trench, which is the most common reason a drain works for two years and then quits.
Which way does the filter fabric face, and do I need a sock on the pipe?
Line the trench with non-woven filter fabric first — walls and floor — then add stone, pipe, more stone, and fold the fabric over the top before backfilling. The fabric is a burrito around the stone, not a layer under the pipe, and the point is to keep soil fines out of the void space. Lap seams 6–12 inches. A fabric sock on the pipe is a second line of defense that helps in sandy or silty soil but clogs faster in clay, so in heavy clay most installers rely on trench-wrapped fabric alone rather than sock plus fabric.
Where does the water actually go at the end?
A French drain only moves water — it has to daylight somewhere lower than the trench, or terminate in a dry well or a storm connection your municipality allows. Never discharge onto a neighbouring property, and never tie into a sanitary sewer; both are illegal in most jurisdictions and can make you liable for the damage downhill. Many towns also regulate discharge near property lines, setbacks, and septic fields. Confirm the outlet with your local building or stormwater department before you dig, and call 811 to have utilities located first.
How much stone does a typical 50 ft French drain take?
A 50 ft run in a 12 in wide by 18 in deep trench is 75 cubic feet of trench. Subtract about 4.4 cubic feet for a 4 inch pipe and you need roughly 2.6 cubic yards, or about 3.7 tons of washed stone at 1.4 tons per yard. Order 10% extra — trench walls are never plumb and the stone settles as you compact around the pipe. If the trench is backfilled with topsoil and sod on top instead of stone to grade, subtract that top layer from the stone volume before ordering.