French Drain Calculator
Washed stone, trench volume and pipe length for a French drain, in yards and tons.
Quick estimating rule of thumb
For a French drain: trench length (ft) × width (ft) × stone depth (ft) ÷ 27 = cubic yards. A 50 ft run 12 inches wide with 18 inches of stone is 2.78 yards, about 3.75 tons. Use washed #57 only, never stone with fines, which compacts solid. Fall must be at least 1 inch per 8 to 10 feet.
This French drain calculator gives the washed stone a drainage trench needs. Enter the trench length as the length, the trench width as the width, and the depth of stone as the depth: a French drain is simply a long thin rectangle, so the geometry is the same as a patio turned on its side.
The stone is the part people underestimate, because a trench is deep. A 50-foot run 12 inches wide and 18 inches deep swallows 2.78 cubic yards, close to 3.8 tons of stone, for a trench you could step over. That is a delivered load, not something to carry home in bags.
How deep and wide should a French drain be?
For surface water and a soggy lawn, a trench 12 inches wide and 18 inches deep is the standard. For a footing drain along a foundation, the trench goes to the base of the footing, which is commonly 24 to 48 inches down, and 12 to 18 inches wide to give room to work. A curtain drain intercepting water on a slope sits 18 to 24 inches deep.
The trench must fall continuously toward the outlet at a minimum of 1%, which is 1 inch of drop for every 8 to 10 feet of run. Below that, silt settles out in the pipe instead of being carried through. Check the fall with a string line and a level before any stone goes in, because it cannot be corrected afterwards.
Which stone, and why washed matters
Use washed #57 stone, 3/4 inch clean, or a similar angular washed aggregate with the fines screened out. The screening is the whole point: the voids between the stones are where the water travels, and roughly 30 to 40% of a clean stone bed is void space. Dense-graded stone with fines in it (road base, crusher run, 3/4 minus) compacts solid and turns a drain into a buried nothing.
Wrap the trench in a non-woven filter fabric, stone and pipe inside it, with a generous overlap at the top. Without fabric, soil migrates into the voids and the drain silts up in a few seasons. Buy fabric by the trench perimeter and add 20%: for a 12 by 18 inch trench that is about 5 feet of fabric width per foot of run.
The build-up in the trench
Bed the pipe on 2 to 3 inches of stone rather than laying it on the trench floor, so water can enter from below. Use 4-inch perforated pipe with the holes facing down, which is counter-intuitive but correct: water rises into the pipe from the surrounding stone and is carried away, rather than draining out of it. Then cover with stone to within 2 to 4 inches of the surface.
The pipe itself displaces stone: a 4-inch pipe takes up 0.087 cubic feet per foot of run, so 50 feet of pipe removes about 4.4 cubic feet, roughly 0.16 cubic yards. On a small job that is inside the rounding; on a 200-foot run it is most of a yard, so subtract it if you are ordering to the half-yard.
Where the water actually goes
A French drain does not make water disappear; it moves it to somewhere lower. Before ordering stone, identify the outlet: daylight on a downhill slope, a dry well sized for the volume, a storm sewer connection if your municipality permits one, or a pop-up emitter in a lawn that is genuinely lower than the trench. A drain built with no outlet is a buried reservoir that fills and stays full.
Discharging onto a neighbouring property is a real legal problem in most jurisdictions, and connecting to a sanitary sewer is prohibited essentially everywhere. If there is no gravity outlet at all, the honest answer is a sump pit with a pump rather than a longer trench, and the stone volume is then whatever the pit needs rather than a 50-foot run.
Common French drain mistakes
Using stone with fines in it. Road base, crusher run and 3/4 minus compact solid. Only washed clean stone keeps the 30 to 40% void space the drain depends on.
Skipping the filter fabric. Soil migrates into the voids and the drain silts up within a few seasons. Line the whole trench with non-woven fabric and overlap it at the top.
Pipe holes facing up. Holes go down, so water enters at the lowest point in the stone bed rather than after the trench has filled to pipe height.
Laying the pipe on the trench floor. Bed it on 2 to 3 inches of stone so water can enter from below as well as from the sides.
No consistent fall. Below 1% (1 inch per 8 to 10 feet), silt settles out inside the pipe and the drain progressively blocks. Set the grade with a string line before any stone goes in.
| Material | lb / ft³ | tons / yd³ | Sold by | Typical use |
|---|---|---|---|---|
| Pea gravel (3/8″, rounded) | 100–105 | 1.35 | ton or yd³ | Paths, patios, dog runs, drainage |
| #57 crushed stone (3/4″, washed) | 100–105 | 1.35 | ton | Drainage, slab sub-base, French drains |
| 3/4″ minus / dense-grade | 105–115 | 1.45 | ton | Driveway base, paver base, road pack |
| Crushed concrete (RCA) | 100–110 | 1.35 | ton | Driveway base, structural fill (cheaper) |
| River rock (1–3″) | 95–105 | 1.40 | ton or yd³ | Dry creek beds, borders, xeriscape |
| Decomposed granite | 110–120 | 1.50 | ton | Paths, patios, natural-look driveways |
| Road base / Class 5 | 110–120 | 1.50 | ton | Compacted base under pavers, slabs, drives |
Background reading: how deep should gravel be.
Frequently asked questions
How much gravel do I need for a French drain?
Multiply trench length by width by stone depth in feet, then divide by 27. A 50 ft run 12 inches wide with 18 inches of stone is 75 cubic feet, or 2.78 cubic yards, about 3.8 tons of washed stone. Add 10% for the trench being wider than you dug it.
What kind of gravel goes in a French drain?
Washed #57 stone or 3/4 inch clean: angular washed aggregate with the fines screened out, so 30 to 40% of the bed is void space for water to move through. Never use road base, crusher run or 3/4 minus; the fines compact and block the drain.
How deep should a French drain be?
Eighteen inches for surface water and lawn drainage, 18 to 24 inches for a curtain drain on a slope, and down to the base of the footing (commonly 24 to 48 inches) for a foundation drain. Depth matters less than continuous fall to the outlet.
How wide should a French drain trench be?
Twelve inches is standard, and 6 to 8 inches is workable for a small lawn drain with a 3-inch pipe. Wider trenches hold more water but cost stone quickly: going from 12 to 18 inches wide adds 50% to the stone order.
What slope does a French drain need?
At least 1%, meaning 1 inch of drop per 8 to 10 feet of run. Set it with a string line before laying stone. Too flat and silt drops out in the pipe; much steeper is fine as long as the outlet is lower than the inlet the whole way.
Do the pipe holes face up or down?
Down. Water rises into the pipe from the saturated stone around it, and holes at the bottom let it in at the lowest possible level. Holes up means water has to fill the trench to pipe height before the pipe does anything.
Do I need filter fabric in a French drain?
Yes, non-woven filter fabric lining the whole trench with an overlap at the top. Without it, soil washes into the voids between the stones and the drain stops working within a few seasons. Buy roughly 5 feet of fabric width per foot of a 12 by 18 inch trench, plus 20%.
How many tons of stone for a 100 foot French drain?
At 12 inches wide and 18 inches of stone, 150 cubic feet, or 5.56 cubic yards, about 7.5 tons of washed #57. That is a full small delivery, so price it by the ton from a quarry rather than by the bag.
Should I subtract the pipe volume from the stone order?
Only on long runs. A 4-inch perforated pipe displaces 0.087 cubic feet per foot, so 50 feet is 4.4 cubic feet, inside the rounding on a small job. On 200 feet it is 17.5 cubic feet, about two thirds of a cubic yard, which is worth taking off.
How much does a French drain cost in materials?
For a 50-foot run: about 3 cubic yards of washed stone at $35 to $55 a yard, 50 feet of 4-inch perforated pipe at $1 to $2 a foot, and a roll of filter fabric, roughly $200 to $300 in material, plus delivery and any excavator hire.
Can I use a French drain without pipe?
A stone-filled trench with no pipe (sometimes called a soakaway trench) works for small volumes over permeable soil, because the voids hold water while it infiltrates. It carries far less water than a piped drain and silts up faster, so for anything draining a downspout or a wet lawn, include the pipe.
How far from the foundation should a French drain be?
An exterior footing drain sits directly against the footing, below the slab level. A curtain drain intercepting surface water should be 4 to 6 feet out from the wall, far enough that the trench does not undermine the foundation but close enough to catch water before it reaches it.
How long does a French drain last?
Thirty years or more when built with washed stone, wrapped in non-woven fabric, and given a real outlet. Built without fabric, or with stone that has fines in it, expect meaningful loss of capacity in five to ten years and a full rebuild not long after.
Should the trench be filled with stone all the way up?
To within 2 to 4 inches of the surface, then either topped with the same stone for a visible drain or capped with soil and turf over the fabric. Do not cap with a compacted soil layer that water cannot get through: that turns a surface drain into a subsurface one.