What the volume arithmetic is doing
A trench is a long box. Its volume in cubic feet is length times width times depth with the inches converted to feet first, and a cubic yard is 27 cubic feet. A hundred foot run, twelve inches wide and eighteen inches deep, is 100 x 1 x 1.5, which is 150 cubic feet, or 5.56 cubic yards. That is the hole. The stone that goes back in is the same volume minus whatever the pipe occupies: four inch pipe over a hundred feet is about 8.7 cubic feet, so the stone is roughly 141 cubic feet, or 5.23 cubic yards.
Stone is sold by weight, not volume, and the conversion is where estimates go wrong. Washed drainage stone commonly falls somewhere around 1.3 to 1.5 tons per cubic yard, and the exact figure depends on the rock, the gradation and how wet it is when it is loaded. That is a wide enough range that 5.23 cubic yards is anywhere between about 6.8 and 7.8 tons. The calculator takes the rate as an input rather than pretending there is one number, and the quarry will give you theirs if you ask.
The fabric wrap is wider than the trench
Filter fabric is not a liner in the bottom of the trench, it is a sock around the stone. Water passes through it and fine soil particles do not, which is what stops the voids in the stone silting up and turning the drain into a buried gravel bar. To do that it has to go up both walls and fold over the top, so the width you need is the trench width plus twice the depth plus enough overlap to close it. A twelve inch wide, eighteen inch deep trench with a twelve inch overlap needs sixty inches of fabric across, which is five feet, and a six foot roll yields exactly one strip.
Check that division before ordering. A roll two feet wider than the strip you need still only makes one strip, so you pay for a third of the roll and use none of it. Narrower trenches sometimes let two strips come off a wide roll, which halves the roll count.
Fall, and what happens without it
Drains work on gravity, and one percent — a foot of drop in a hundred feet — is the number people generally aim at. Less than that is buildable but unforgiving; the trench bottom has to be genuinely graded rather than roughly sloped, and any settlement creates a flat spot that collects silt. More than one percent is fine as far as the water is concerned, but it drives the outlet end deep fast, and every extra inch of depth is more excavation, more stone and a worse trench to be standing in.
| Slope | Drop per 100 ft | Where it sits |
|---|---|---|
| 0.5% | 6 in | About the practical minimum for a graded trench bottom |
| 1% | 12 in | The common target for a residential run |
| 2% | 24 in | Fine hydraulically, expensive in excavation over a long run |
The drain slope calculator works the same arithmetic in the other direction if what you have is a fixed outlet elevation and a question about whether the fall exists at all. That is usually the honest order to do it in: find the outlet first, then see what slope the site allows, then decide how long the run can be.
Before the shovel
Before any of this turns into a shovel in the ground: a free national utility-locate service operates in every US state, and calling it is how buried gas, electric, water and communication lines get marked. It costs nothing. Striking a gas main or an electric service is not an inconvenience or a repair bill, it is a life-safety event, and it happens most often on shallow residential digs exactly like this one. Look up the locate service for your state, request the marks, and wait until they are on the ground before you dig.
Where the water comes out is a legal question as much as a drainage one. Discharging onto a neighbouring property, into a street, into a ditch or into a municipal storm system is governed by local rules that vary between towns and sometimes between parcels, and easements, wetland setbacks and stormwater ordinances can all apply to a residential drain. Nothing on this page is a permission. Ask the local building department or the stormwater authority what your outlet is allowed to do before you build toward it.
What a French drain does not fix
A subsurface drain intercepts water moving through soil and water sitting on it. It does not fix a grade that falls toward the house, a downspout emptying against the foundation, or a gutter that has been overflowing for two years. Those are cheaper problems and they are the actual cause a surprising amount of the time, which is why the gutter and downspout sizing calculator is worth ten minutes before you price out seven tons of stone. If the water is already inside and being pumped, the sump pump sizing calculator covers the other end of the same system.
It also does not fix soil that will not accept water. A drain in dense clay with nowhere to discharge is a trench full of stone holding whatever fits in the voids, and then overflowing. The question worth answering before you dig is not how much stone, it is where the water goes after it enters the pipe.
Questions people ask
How many tons of gravel do I need for a French drain?
Work out the volume first, then convert. Trench length times width times depth, with the inches turned into feet, gives cubic feet; divide by 27 for cubic yards; subtract what the pipe occupies. Then multiply by the tons-per-cubic-yard figure for the stone you are actually buying, which for washed drainage stone is commonly somewhere around 1.3 to 1.5. A 100 ft trench at 12 by 18 inches with 4 inch pipe works out near 5.2 cubic yards, which is roughly 6.8 to 7.8 tons depending on the stone. The spread between those two numbers is a full pickup load, so get the real figure from the supplier rather than using a rule of thumb.
Does a French drain need pipe, or is stone enough?
Stone alone drains, but slowly, because water has to move through the voids between the stones for the whole length of the run. A perforated pipe in the bottom gives the water an open channel once it has entered the stone, which is why almost every modern installation has one. The pipe also gives you something to jet or rod if the drain silts up years later, which a solid stone trench does not. The case for stone alone is short runs, tight sites, or an interception trench right beside a wall where the water only has to travel a few feet.
Which way do the holes face?
This gets argued about endlessly and the argument matters less than the fabric. The common practice is holes down, on the reasoning that water enters at the lowest point in the trench and the pipe fills from the bottom, so the drain begins working before the water table rises to the top of the stone. Holes up is defended on the grounds that silt is less likely to enter. Either orientation drains. What actually determines whether the drain is still working in ten years is whether the stone is properly wrapped in filter fabric and whether the trench has real fall.
How deep should the trench be?
Deep enough to intercept the water you are trying to catch, which is a site question rather than a number. A drain catching surface water and shallow saturation might sit twelve to eighteen inches down. One relieving hydrostatic pressure against a foundation wall sits at or near the footing, which is a different scale of excavation and a different set of risks. Depth also compounds: at one percent fall a long run gets deep at the outlet end whether you wanted it to or not. Trench walls collapse without warning, and a collapse with a person in the trench is how excavation work kills people. Past about four feet this stops being a weekend job.
Where is the drain allowed to discharge?
That is a local question with a legal edge, and it is not one this page can answer. Discharge onto adjoining property, into a street or ditch, or into a municipal storm system is regulated differently from town to town, and easements, wetland setbacks and stormwater rules can all apply to a single residential run. Concentrating water and directing it onto a neighbour is also the kind of thing that generates disputes independently of the code. Ask the local building department or the stormwater authority what your intended outlet is allowed to do, and settle that before you commit to a route.