How much gravel for a french drain?
Gravel fills the trench minus the pipe. A standard 12 in wide, 18 in deep trench needs about 1.4 cu ft of stone per linear foot with a 4 in pipe. That’s roughly one 50 lb bag per 4 inches of run, so bulk delivery usually wins on anything past about 20 feet.
Getting the fall right
A french drain needs at least 1% slope, which is 1/8 inch of fall per foot for 3–6 in pipe. Set the outlet first and work back uphill. Line the trench with non-woven geotextile, lapped 12 in at the top, so soil never reaches the stone.
How much gravel for a 100 ft French drain, and for 50 ft and 200 ft
Take the section most yard drains are built to. That’s 100 ft long, 12 in wide, 18 in deep, with 4 in corrugated pipe bedded in the bottom. The raw trench is 100 × 1 × 1.5 = 150 cubic feet. The pipe occupies 11 cubic feet of that, leaving 139 cubic feet of stone.
Then add the ten percent waste this French drain gravel calculator carries by default (over-dig, the stone that vanishes into soft trench walls, the wheelbarrow-load you spill). That brings you to 153 cubic feet, or 5.66 cubic yards, which is 7.9 tons at 1.4 tons per yard.
The same 100 ft run also needs 100 ft of perforated pipe and 500 sq ft of filter fabric, which is the part most people forget to price.
Halve the run and everything halves with it. A 50 ft drain at the same section wants 2.83 cubic yards, just under 4 tons, and 250 sq ft of fabric. Double it and 200 ft takes 11.3 cubic yards, close to 16 tons, and 1,000 sq ft of fabric.
The bag count is what should change your mind about how to buy it. That 100 ft drain works out at 306 half-cubic-foot bags at roughly 50 lb each. That’s over seven tons carried by hand, three bags at a time, at several times the bulk rate. Bagged stone earns its keep on a 10 or 15 ft spur behind a downspout, or where no truck can reach the trench. Past about 20 ft, order bulk.
Settle where the water goes before you order, because the outlet decides the length you’re pricing. Daylighting onto a bank or into a ditch is the simplest and cheapest ending.
Where there’s nowhere to daylight, the run finishes in a dry well, and that’s a second stone order rather than a rounding error. Clean washed stone stores water in its voids at roughly forty percent. So holding the 623 gallons that one inch of rain drops on 1,000 sq ft of roof needs something like 208 cubic feet of aggregate, a pit around 6 ft square and 6 ft deep. Plan the whole path on paper first, then run the numbers once for the trench and once for the well.
Why the pipe displaces gravel, and what most French drain calculators miss
Plenty of drainage gravel calculators multiply length by width by depth and stop there. That over-orders, because the pipe is sitting in the trench occupying volume that stone would otherwise fill. A 4 in pipe with an outside diameter of 4.5 in displaces π × (4.5 ÷ 24)² = 0.11 cubic feet for every foot of run.
Over 100 ft that comes to 11 cubic feet displaced, or 0.41 of a cubic yard. Call it a little over half a ton of stone you’d otherwise pay for, haul, and then have sitting on the driveway in a pile. On an eight-ton order it isn’t catastrophic, but it’s real money and a genuinely annoying leftover, so this tool subtracts it before it applies waste.
The two pipe options are closer in volume than their labels suggest. Single-wall corrugated pipe sold as 4 in measures roughly 4.5 to 4.75 in across the ribs depending on who made it, and the manufacturers won’t pin it down for you. ADS publishes the outside diameter of its single-wall pipe as a reference value only, carrying a stated tolerance of half an inch or more. That’s because AASHTO M252, the standard it’s built to, fixes the bore and leaves the outside a range.
ASTM F405 does the same. This tool works from the bottom of that range, 4.5 in. The same pipe is sold in Canada as weeping tile, most often Big O, with or without a filter sock.
The rigid option here is sized on 4.215 in, the outside diameter of ASTM D2729 PVC sewer-and-drain pipe. That’s the thin-wall white perforated pipe most landscape yards stock.
Schedule 40 is the heavier-walled pipe at 4.500 in OD, the same outside diameter as the corrugated option. The difference between the two figures is about 1.4 cubic feet over 100 ft, under a tenth of a ton, and comfortably inside your waste allowance either way.
Choose no pipe at all and the displacement drops out, so a pipe-less trench asks for more stone rather than less. The same 100 ft run climbs from 7.9 to 8.6 tons. A blind drain (trench, fabric, stone, no pipe) moves water through the voids between the aggregate alone. It carries a small fraction of what a 4 in pipe carries and silts up sooner, because there’s nothing you can ever flush.
It’s a fair choice for a short interceptor across a slope, or an infiltration strip beside a patio. For anything that has to shift real volume, or anything you’d rather not dig up in five years, put the pipe in.
Tons per cubic yard of #57 stone: why 1.35 and 1.5 both get quoted
The 1.4 tons per cubic yard this calculator uses is a judgment call rather than a constant. Loose washed drainage stone is generally quoted between 95 and 105 lb per cubic foot, which works out at 1.28 to 1.42 tons per cubic yard. Dense limestone or trap rock, and any stone still carrying water after rain, sits above that, and every #57 stone calculator you compare against will give you a slightly different tonnage.
Neither number is wrong. They come from different quarries, different rock and different weather. So one supplier’s site says 1.35 tons per cubic yard and the next says 1.5.
Run the 100 ft example through the whole range and the spread is real but manageable. Those 5.66 cubic yards come out at 7.6 tons using 1.35 and 8.5 tons using 1.5, against the 7.9 the tool reports. A free cross-check is the bag count. The 306 bags at 50 lb come to 7.65 tons, which lands at the bottom of the band because bagged stone is priced on a 100 lb per cubic foot assumption.
The practical lesson is to order by the cubic yard wherever the supplier allows it. Yardage is a measured volume you can verify in the trench. Tonnage is a conversion, and the conversion belongs to the quarry. Ask them what their #57 weighs; the scale house will know.
The same variability bites harder on storage volume than on tonnage. Designers routinely credit clean #57 with forty percent void space. Contech’s published analysis of that assumption puts it at only about sixty percent reliable, and recommends thirty-six percent for a conservative design. That lower figure reaches ninety-six percent reliability once you allow for how much gradation varies from quarry to quarry, and even between pits owned by the same company.
Sediment occlusion is a separate loss stacked on top of that. Contech’s blunt version is that one month of poor erosion control during construction costs you more void space than years of silt off a finished paved surface. Use the lower figure if you’re sizing a dry well rather than a trench.
On price, #57 commonly runs somewhere around $25 to $75 a ton across the US as of 2026, depending on region and rock type. Picked-up quarry loads sit at the bottom of that band and landscape-yard retail at the top, with haulage charged per load on top of either, which is why a two-ton order looks extortionate per ton and a full 15 to 20 ton load looks cheap.
Gravel per linear foot for 9, 12 and 18 in wide trenches
Once the section is fixed, the stone figure per linear foot never changes. It’s worth carrying in your head when you’re standing in a yard deciding how wide to dig. At 18 in deep with a 4 in pipe, a 9 in wide trench takes 1.01 cubic feet of stone per foot of run, a 12 in trench takes 1.39, and an 18 in trench takes 2.14. Deepen the 12 in trench to 24 in and it rises to 1.89 cubic feet per foot.
Width costs far more than people expect. Converted at 1.4 tons per cubic yard and before waste, that’s roughly one ton per 19 ft of the narrow section, per 14 ft of the standard one and per 9 ft of the wide one.
How deep a French drain should be is really two questions. For general yard drainage, 18 to 24 in deep and 9 to 12 in wide is the working range. That’s deep enough to intercept the shallow water that keeps a lawn soft, and shallow enough to dig without shoring or a machine.
Against a house you go to the footing, and the footing sits at or below the frost line. That’s roughly 36 in through the mid-Atlantic and Southeast, 48 in across southern Ontario, 60 to 84 in through the northern states, Ottawa and the Prairies, and as little as 24 in on the coast of British Columbia. Check your local frost depth before you price the dig. In freeze-thaw country, make sure the run daylights or self-drains, because a trench that holds water at 18 in freezes solid for months.
On what goes in the trench, the US residential code is specific. Gravel or crushed stone drains must extend at least 1 ft beyond the outside edge of the footing and 6 in above the top of it. The perforated pipe sits bedded on not less than 2 in of washed stone, covered by not less than 6 in more of the same material.
Canada works to NBC/OBC 9.14.3 instead. Pipe must be not less than 100 mm (4 in) diameter, laid so the top of the pipe sits below the underside of the floor slab. It’s covered on top and at the sides with at least 150 mm (6 in) of crushed stone carrying no more than 10 percent passing a 4 mm sieve. There’s no equivalent of the 1 ft beyond the footing rule.
Depth is also governed by fall, and no French drain trench calculator can work that out for you. The outlet must be the lowest point, so set that first and work back uphill at your chosen grade. One eighth of an inch per foot is the one percent the plumbing code sets as the minimum for 3 in and larger drain pipe, and the figure drainage contractors carry over to French drains. How much fall is that over a real run? At that grade a 100 ft run drops just over 12 in, so a trench starting 18 in down finishes around 30 in down.
Some contractors argue for a quarter inch per foot on 4 in perforated pipe to keep silt moving. Take it if you have the fall, and price the extra depth before you dig.
What #57 stone is, and the filter fabric that goes with it
"#57" is a label for a gradation rather than a single stone size, and it’s defined in ASTM D448. In plain terms it’s mostly ¾ to 1 in stone with the fines screened out, which is precisely the point.
AASHTO M43 carries an identical envelope. The specification is 100 percent passing the 1½ in sieve, 95 to 100 percent passing 1 in, 25 to 60 percent passing ½ in, 0 to 10 percent passing the No. 4 and 0 to 5 percent passing the No. 8. The sand and dust that would otherwise pack into the voids simply aren’t in the load.
Ask a yard for washed #57 by name and they’ll know what you mean, whatever rock it happens to be crushed from. Canadian yards don’t use the ASTM numbers at the counter. Ask for 3/4 in clear stone, sometimes listed as 19 mm clear, which is the same washed, fines-free material.
That gradation is why #57 is the best gravel for a French drain, and why rounded pea gravel is the wrong choice even though it drains happily in a bucket. Angular faces lock against one another and hold an open skeleton under load. Rounded stone rolls into its tightest packing as you backfill and compact over it, leaving smaller voids and a tendency to migrate along the trench.
The code language points the same way, requiring stone at least one sieve size larger than the perforations so fines can’t wash in through the holes. Whatever grade you settle on, it must be washed. Unwashed crusher-run belongs under a driveway, never in a drain.
Filter fabric width falls straight out of the geometry. The sheet runs down one wall, across the bottom, up the other wall and then laps over itself at the top. So the width you need is trench width plus twice the trench depth plus your overlap. A 12 in wide, 18 in deep trench works out at 12 + 36 + 12 = 60 in, or 5 ft.
Non-woven geotextile is stocked at the big-box stores in 3 ft × 300 ft and 6 ft × 300 ft rolls. The 3 ft is too narrow for the 5 ft a 12 × 18 in trench needs, so buy the 6 ft. Contractor rolls of 12.5 ft × 360 ft and 15 ft × 300 ft exist if you’re lining several runs.
Buy non-woven, not woven weedblock. Around 4 to 6 oz is the usual weight for yard work. AASHTO M288, the standard governing subsurface-drainage geotextiles, specifies grab strength, permittivity and apparent opening size rather than ounces. Class 2 covers most ordinary installations, and the oz figure is only trade shorthand.
French drain for a 50 ft run: stone, pipe and fabric
A 50 ft run at the standard section is 75 cubic feet of trench. (Standard here means 12 inches wide, 18 inches deep, with 4 in pipe bedded in the bottom.)
That’s 2.78 cubic yards of washed stone, about 3.75 tons, or nearer 2.6 yards and 3.5 tons once the pipe displaces its share. Add 50 ft of perforated pipe and 250 square feet of filter fabric. Budget 6-1/4 inches of fall end to end.
At the $55 a ton this calculator defaults to (a 2026 US figure, your yard will quote its own), that’s about $205 of stone. It’s comfortably past the point where bagged material stops making sense, since the same volume is 150 half-cubic-foot bags to carry by hand.
People budget for the stone and forget the spoil. You’re digging 75 cubic feet out of the ground, and excavated soil swells about a quarter, so plan for roughly 3.5 cubic yards of loose material to haul away or spread. Two people with a rented trencher will usually run 50 ft in a day including fabric and backfill. The same trench by hand in clay is a weekend. Settle where the water is going before you price any of it, because the outlet sets the length.
#57 vs #4 stone for a French drain
Think of #57 as the default and #4 as the specialist. Under ASTM D448, #57 is nominally 1 inch down to the No. 4 sieve, while #4 runs 1-1/2 to 3/4 inch. The bigger stone gives bigger passages and shrugs off silt for longer. It’s also brutal to shovel, impossible to walk on, and not stocked everywhere.
The gain is smaller than the size difference suggests. Both are open-graded and both land in the same rough 35 to 40 percent void band designers work with, so what changes is the size of the individual passage rather than the total storage. That makes #4 worth specifying in deep, high-flow trenches, or as a bottom lift under #57 where the trench bottom is soft and #57 alone would press into it.
Two conditions come with it. Fabric isn’t optional, because soil drops straight into voids that big, and your pipe perforations must be smaller than the stone so nothing washes in through the holes.
Unless you’ve got a real reason to go bigger, stick with it. For an ordinary yard drain, #57 keys together better, caps more neatly and is on the yard’s shelf today.
French drain slope: 1 inch per how many feet?
One inch of fall every 8 feet. That’s the 1 percent minimum drainage contractors work to, which comes to 6-1/4 inches over a 50 ft run and 12-1/2 inches over 100 ft. Steeper is fine (a quarter inch per foot is 1 inch every 4 feet), but a flat stretch or a belly in the middle isn’t.
Set it with string, not by eye. Drive a stake at each end and run a taut line with a line level on it. Then drop the downhill end by the total fall you calculated, and measure from the string down to the trench bottom every 10 feet as you dig. A rotary laser does the same job faster on long runs.
Over-dig slightly and bed the pipe on 2 inches of stone, so you can fine-tune the grade by raking rather than by re-digging.
Before any fabric goes over the top, run a hose in at the high end and watch. Water that stalls or pools tells you where the belly is while it’s still cheap to fix.
French drain depth in clay soil
Keep it to 18 to 24 inches unless you’re draining a footing. Clay moves water sideways so slowly that a deep trench collects almost nothing from the soil around it. So what feeds the drain? Mostly water arriving from the surface and through the looser topsoil sitting above the clay.
The drainage research puts numbers on it. NDSU’s guidance shows a heavy Fargo clay needing laterals about 40 ft apart at a 4 ft depth to do the same job that a fine sandy loam achieves at around 120 ft. In clay, each run only serves a narrow band either side of itself. Practically that means more runs, placed where the water shows up, rather than one deep trench doing nothing expensive.
Bring the stone up to within a couple of inches of the surface, or cap it with more stone instead of sod, so surface water has a route in.
A clay trench sealed under topsoil is fed by almost nothing. Wrap the whole section in non-woven fabric too, because clay fines are exactly what blinds a drain.
French drain vs dry well: which one does your yard need?
A French drain moves water somewhere else. A dry well holds it until the ground can take it. Use the drain when you have an outlet to run to, the well when you don’t, and both together when the trench has nowhere to daylight. In clay, the drain works and the dry well usually doesn’t.
Test before you dig the pit. Fill a hole with water and time it. Below about half an inch an hour of infiltration you’re building a bathtub, not a dry well.
Size it against real rainfall. One inch of rain on a 400 sq ft roof plane is 249 gallons, which is 33 cubic feet of water. At a conservative void ratio that means 90-odd cubic feet of stone, or a pit roughly 4 × 4 × 6 ft. Prefabricated chambers change the arithmetic rather than the volume. An NDS Flo-Well is a 24 in diameter, 24 in tall unit holding 50 gallons, so that same downspout wants about five of them. Keep any well at least 10 ft out from the foundation and downhill of it.
Common questions
How much gravel per foot of french drain?
Figure on about 1.4 cu ft per linear foot for a 12 × 18 in trench with 4 in pipe. Call it one ton per 14–15 ft of run, give or take the quarry’s density.
Can I use pea gravel?
Better not. Rounded pea gravel packs tight and impedes flow. Use washed angular crushed stone, 3/4 in ASTM #57 in the US or 3/4 in clear stone in Canada.
How deep and wide should the trench be?
For most yard drains, 12 in wide and 18 in deep is about right. Alongside foundations you’ll usually want to go 24 in or deeper, down toward the footing.
How much slope does it need?
At least 1%, which is an eighth of an inch per foot. Any less and silt tends to settle inside the pipe instead of washing through to the outlet.
Do I need fabric and a pipe?
Fabric, always — it keeps soil out of the stone. A pipe-less ‘blind’ drain works only for short runs with modest water.
How many tons of gravel do I need for 100 feet of French drain?
Roughly 7.9 tons for the standard section, meaning 100 ft of 12 in wide, 18 in deep trench with 4 in pipe, including ten percent waste. That’s 5.66 cubic yards, or 306 half-cubic-foot bags. Widen the trench to 18 in and it climbs to roughly 12 tons. Narrow it to 9 in and it drops to under 6.
Is #57 stone 1.35 or 1.5 tons per cubic yard?
Both figures get quoted and both are defensible. Loose washed stone runs roughly 95 to 105 lb per cubic foot, which is 1.28 to 1.42 tons per cubic yard, with dense or wet rock above that. This tool uses 1.4. Order by the cubic yard where you can, and ask the quarry what their #57 actually weighs.
How wide does the filter fabric need to be?
Trench width plus twice the trench depth plus your overlap. A 12 in wide, 18 in deep trench needs 60 in (5 ft) to line both walls and the bottom and still lap 12 in at the top. Use 4 to 6 oz non-woven geotextile. Big-box rolls are 3 or 6 ft wide, so buy the 6 ft. The 12.5 and 15 ft contractor rolls only pay off across several runs.
Does a French drain need a clean-out?
It’s cheap insurance on any run you’d hate to dig up. A 4 in riser teed into the line lets you jet the pipe with a hose instead of excavating. Common practice is one at the head of the run, one at any bend sharper than 45 degrees, and one every 50 to 75 ft along long straights.
Can I run my downspouts into the same French drain?
You can, and it’s common, but size the outlet for both loads. One inch of rain on 1,000 sq ft of roof is 623 gallons arriving in minutes, far more than the groundwater the trench was dug for. Keep roof water in solid pipe until it joins the perforated run, and add a dry well if there’s nowhere to daylight.
Should the pipe perforations face up or down?
Down, at roughly the four and eight o’clock positions. Water collects at the bottom of the stone bed, so downward holes begin draining at the lowest water level instead of waiting for it to rise. A minority of designers argue holes up reduces silting, but holes down is the mainstream trade practice, and it’s the reason for the bedding stone underneath.
How far from the house should a French drain be?
Depth matters more than distance. An interceptor drain catching water before it reaches the house usually sits 4 to 6 ft out. A footing drain sits against the footing itself. What decides performance is the invert elevation. It needs to be below the level you’re willing to let groundwater rise to, which for a basement means at or under the bottom of the footing.
What size dry well do I need for one downspout?
For a 400 sq ft roof plane, an inch of rain is 249 gallons, about 33 cubic feet of water, or 90 to 95 cubic feet of clean stone once you allow for void space. That works out to a pit around 4 × 4 × 6 ft, or five 50-gallon chambers. Many jurisdictions size to a specific design storm, so find out what yours expects before you dig.
What if my yard has no slope for a French drain?
You’ve got three options, in order of cost. Regrade the surface so water runs to a lower corner, terminate the run in a dry well if the soil percolates, or dig a sump basin at the low end and pump the discharge up to daylight. What you must not do is build a level trench and hope. Standing water in the stone silts the pipe and freezes in winter.