A French drain is a gravel-filled trench, usually with a perforated pipe at the bottom, that collects water from the surrounding ground and moves it somewhere it can do no harm. This calculator works out everything you have to buy for one: the excavation, the stone tonnage with the pipe volume taken out, the filter fabric including the overlap, and the fall along the run.
Arb Digital publishes this as a planning and take-off tool. Our general gravel calculator covers loose aggregate over an area, which is the right tool for a driveway or a bed. This one is a trench: it subtracts the volume the pipe occupies, sizes the fabric that wraps the stone, and checks whether the run actually falls, none of which an area calculation does.
What This French Drain Calculator Does
It treats the drain as a rectangular trench of a given length, width and depth. From that it computes the excavation volume, then the stone envelope, which is everything below the cap. It converts the pipe outside diameter into the volume of stone the pipe displaces along the run and subtracts it. It works out the fabric width needed to line the trench and lap over the top, multiplies by length, and gives the roll area. Finally it applies the fall percentage to the run length to give the total drop, which is the number that decides whether the outlet is where you thought it was.
The stone density is an input, not a built-in constant. Aggregate density varies with the rock type, the grading, the shape of the particles and how wet it is. Washed angular stone of the kind used in drains is lighter per unit volume than dense-graded crushed material, because it has a large void fraction — which is the point, since those voids are the storage. Get the figure from the supplier for the exact product, and if the yard sells by volume rather than weight, use the volume result and ignore the tonnage.
How to Use It
- Measure the run along its path. Curves and dog-legs add length and they add cost proportionally.
- Set trench width and depth. Depth is driven by where the water is and where the outlet can be, not by a rule of thumb.
- Give the cap depth. Zero for an open stone drain, or the topsoil and turf thickness for a hidden one.
- Enter the pipe outside diameter. Nominal sizes understate the outside of most corrugated pipe, and it is the outside that displaces stone.
- Set the fall and check the total drop. If the drop is more than the difference in level between the two ends of the run, the design does not exist yet.
The Formula and How It Is Calculated
Excavation is length × width × depth. Stone envelope is length × width × (depth − cap). Pipe displacement is π ÷ 4 × outside diameter squared × length. Net stone volume is the envelope minus the displacement, and tonnage is that volume multiplied by the density you entered.
Fabric width is the trench width plus twice the stone depth plus your overlap allowance, because the fabric goes down one wall, across the base, up the other wall and then laps over the top. Fabric area is that width multiplied by the run length. Total fall is the run length multiplied by the fall percentage.
Worked example against the loaded values. A 100 foot run, 1 foot wide and 2 feet deep, is 200 cubic feet of excavation, which is 7.41 cubic yards. With a 3 inch cap the stone envelope is 175 cubic feet. A 4 inch outside diameter pipe displaces π ÷ 4 × 0.333² × 100 = 8.73 cubic feet, leaving 166.3 cubic feet of stone, or 6.16 cubic yards. At 1.4 tons per cubic yard that is 8.62 tons. The fabric width is 1 + 3.5 + 1 = 5.5 feet, so 550 square feet over the run. At one per cent, the drain falls 1 foot from end to end. Excavated at 25 per cent swell, the 7.41 cubic yards in the ground becomes 9.26 loose cubic yards of spoil to move.
The Pipe Volume Is Small, and It Is Not the Point
In the worked example the pipe takes out about five per cent of the stone. On a small job that is inside the rounding when you order by the ton. So why calculate it?
Because the ratio changes fast. Put a 12 inch pipe in an 18 inch wide trench and the pipe is displacing a serious fraction of the envelope. Run several pipes in one trench and it grows again. And on a long run the absolute number stops being trivial: the same five per cent over 800 feet is most of a truckload of stone you would otherwise have paid to dump on a driveway.
The wider point is that a French drain take-off is a volume problem with a hole in it, and area-based calculators cannot see the hole. Where the trench itself is what you are pricing, our soil volume calculator handles bulk earth volumes and the slope calculator converts between fall expressed as a percentage, a ratio and an angle.
Fall Is the Thing That Decides Whether It Works
Almost every French drain that fails to perform fails on fall rather than on materials. Water moves through the stone because it is going downhill, and if the outlet is not lower than the collection point the drain simply fills and stays full.
The total drop matters more than the percentage. A run at one per cent over 100 feet drops a foot. Over 300 feet it drops three feet, and the outlet has to be three feet lower than the head of the drain. On a flat site that is often physically impossible, and the honest conclusion is that this drain cannot be gravity-fed along that route, so it needs a shorter run, a different alignment, a sump, or a different solution altogether.
The other half of the same problem is what happens at the outlet. A drain has to discharge somewhere that can take the water: a swale, a dry well, a storm system where the local authority permits a connection, or daylight on a lower part of the site. Discharging onto a neighbour's land is a dispute and, in many jurisdictions, an offence. Connecting to a sanitary sewer is generally prohibited outright. The EPA Soak Up the Rain programme is a good non-technical starting point on managing runoff on a property, and the USDA Natural Resources Conservation Service publishes the soil survey data that tells you how the ground on your site actually drains.
Filter Fabric, Voids and Why Drains Silt Up
The stone in a French drain is not doing any filtering. It is doing two jobs: holding open a large void volume so water can move faster than it could through soil, and keeping the trench from collapsing. The filter fabric is what stops fine soil particles migrating into those voids.
When a drain stops working after a few years, silt is usually the reason. Fines wash in from the sides, fill the voids between the stones, and the trench becomes a long thin lump of dirty gravel with the hydraulic conductivity of the surrounding soil. That is why the fabric wraps the whole envelope rather than just sitting on top, and why the overlap matters: an unlapped seam is a doorway.
It is also why the stone should be washed and single-sized rather than dense-graded. Dense-graded material is designed to compact and interlock with very few voids, which is exactly wrong here. And it is why the pipe is laid with the perforations positioned according to the manufacturer's instructions rather than by folklore — the pipe is a conveyance at the bottom of a stone reservoir, and where its holes face changes how it fills.
Sizing the Drain to the Water, Not to the Trench
This calculator sizes materials for the drain you describe. It does not tell you whether that drain is big enough for the water arriving, which is a hydrology question with a different set of inputs: the contributing catchment area, the rainfall intensity for a chosen design storm, the infiltration rate of the soil and the outfall capacity.
Those numbers are local. Rainfall intensity comes from published data for your region, soil infiltration from a percolation test on the actual site, and any discharge to a public system from the authority that owns it. Our rainfall volume calculator turns a depth of rain over a catchment into a volume of water, which is a useful first sanity check on whether the storage in your stone envelope is in the right order of magnitude. For anything that protects a building, or for any site where the water has to cross a boundary, this is drainage design and belongs with a civil engineer or a drainage contractor working to local requirements.
Arb Digital builds free calculators and interactive tools that earn search traffic for landscaping, groundworks and construction businesses. Browse the full library, or tell us what your customers keep asking you to work out.
Browse Free Tools Talk To Arb DigitalCommon Mistakes to Avoid
- Building a drain with nowhere to go — without an outlet lower than the collection point, a French drain is a buried reservoir that fills once and stays full.
- Using dense-graded aggregate — it is designed to compact with minimal voids, and the voids are the entire reason the stone is there.
- Skipping or under-lapping the fabric — fines migrating into the stone is the standard failure mode, and an open seam is where they get in.
- Ordering stone by trench volume — the pipe displaces part of the envelope, and on a large pipe in a narrow trench that is a significant share.
- Forgetting the spoil — excavated soil bulks up on the way out of the ground, and it has to be carted away or placed somewhere on site.
Related Free Tools From Arb Digital
Pair this with the gravel calculator for aggregate spread over an area, the soil volume calculator for bulk earthworks, the slope calculator for converting fall between percentage, ratio and degrees, the rainfall volume calculator for how much water a catchment sheds and the pipe flow calculator for what the outfall pipe can carry. The full free online tools hub lists every calculator we publish.
Frequently Asked Questions
The gravel calculator spreads aggregate over an area to a depth, which suits driveways and beds. This one is a trench take-off: it subtracts the volume the pipe displaces, sizes the filter fabric including the overlap and checks the fall along the run.
The tool does not prescribe one, because it depends on the site and on local drainage requirements. What matters more than the percentage is the total drop it implies over your run length, and whether the outlet can physically sit that much lower than the head of the drain.
Because the pipe occupies space that would otherwise be stone. On a small pipe in a wide trench that is a few per cent, but a large pipe in a narrow trench displaces a substantial share of the envelope, and over a long run the difference is measured in truckloads.
The one your supplier quotes for the exact product. Density varies with rock type, grading, particle shape and moisture, and washed single-sized drainage stone is lighter per unit volume than dense-graded material because of its void fraction. No density is published here for that reason.
It is what stops fine soil migrating into the voids between the stones. Silting is the usual reason a drain that worked at first stops working after a few years, and the fabric wraps the whole envelope with a lapped seam precisely to close that path.
No. Where a drain may legally discharge is set locally, discharging onto adjoining land causes disputes and is often an offence, and connections to public systems need the permission of whoever owns them. Sanitary sewer connections are generally prohibited outright.
No. Material take-off and hydraulic capacity are different questions. Sizing a drain to the water arriving needs the catchment area, a design rainfall intensity, a measured soil infiltration rate and the outfall capacity, which is drainage design work.
Because soil swells when it is dug. In-place ground is compacted; once broken up it occupies more space, so the loose volume you cart away is bigger than the hole you made. The swell percentage in the tool converts between the two.
This tool is a material take-off and planning aid using dimensions and a stone density you supply. It is not a drainage design, it does not size a drain to the water arriving, and it publishes no aggregate density figures. Drainage that protects a building, crosses a boundary or connects to a public system should be designed by a qualified engineer or drainage contractor to local requirements.
