French Drain & Drainage Calculator
A French drain is a gravel-filled trench — usually wrapped in filter fabric and holding a perforated pipe at the bottom — that collects groundwater and carries it away from a low spot, a foundation, or a soggy patch of yard. Getting the materials right means four separate take-offs at once: how much drainage stone to order, how long a run of perforated pipe you need, how much filter fabric wraps the trench, and whether the trench falls fast enough to actually drain.
This calculator turns each trench segment's length, width, depth, pipe size and grade into all four numbers — net drainage gravel (with the pipe volume subtracted), perforated pipe length, filter-fabric area, and the slope check with total fall — plus the trench spoil you dig out and an optional cost. Everything is computed in your browser; nothing you enter is sent to a server.
Add a segment for each run — a single straight drain, a wrap around a foundation, or a perimeter of several legs — pick a profile preset or type your own dimensions, and the results rail totals the whole job as you type.
How to use the French drain calculator
Choose metric or imperial units at the top; the calculator remembers your choice in your browser. Metric works in metres, cubic metres and tonnes; imperial in feet, cubic yards and short tons, with linear dimensions in inches.
Add a segment per run of trench, then set its length, width and depth. Pick a profile preset — yard, foundation, curtain or driveway — to seed a typical width, depth and pipe size, then edit any field freely. The driveway preset is a typical starting point rather than a standard, so sanity-check it against your own job.
Choose a pipe diameter (or 'None' for a gravel-only, pipeless drain), set the grade, and read the results rail. It totals drainage gravel volume, weight, bags and truckloads, perforated pipe length, filter-fabric area and rolls, trench spoil, and — once you enter any price — a line-item cost.
How to size a French drain in 5 steps
- Choose unitsSelect metric (m, m³, tonnes) or imperial (ft, yd³, short tons) at the top of the page.
- Add segmentsAdd one segment per run of trench. Pick a profile preset (yard, foundation, curtain or driveway) to seed a typical width, depth and pipe size, then enter the run length and adjust any dimension.
- Set pipe and gradeChoose a pipe diameter — 3", 4" or 6" (75, 100 or 150 mm) — or 'None' for a pipeless gravel drain, then set the grade as a percentage or as fall per run.
- Set materials and cost (optional)Adjust the drainage stone, fabric overlap, outlet-stub allowance and roll width, and add prices for gravel, pipe, fabric and a flat fee to see an estimated cost.
- Read the resultsThe rail totals drainage gravel volume, weight, bags and truckloads, perforated pipe length, filter-fabric area and rolls, trench spoil, a below-minimum-grade warning, and optional cost.
How French-drain sizing works
Drainage gravel = trench volume − pipe volume. The trench is length × width × depth; the perforated pipe is a cylinder (π × radius² × length) sitting inside it. The calculator subtracts the pipe's solid volume so you order gravel for the space the stone actually fills, not the whole trench. A pipeless drain skips the subtraction — the trench is gravel all the way down. A waste allowance is then added on top for spillage and over-dig, and the result converts to weight, bags and truckloads.
Filter fabric = length × (width + 2 × depth + overlap). One continuous sheet lines the trench floor and both walls, then folds back over the top of the stone — that top fold is the overlap term (6 in / 150 mm is a typical, editable default). Under-ordering fabric is one of the most common French-drain mistakes, because the wrap has to cover far more than the trench opening. If you enter a roll width, the calculator also returns a rolls-needed count.
Grade and fall. A drain only works if it falls steadily toward its outlet. The widely cited minimum is about 1% — that is 1 inch of fall for every 8 feet of run, or 10 mm per metre. Total fall = run length × grade, so a 40 ft run at 1% drops about 4.8 inches over its length. The calculator flags any segment below 1% with a warning. These are commonly required minimums — check local code for the figure that applies to your job; the tool is a planning aid, not a code determination.
Frequently asked questions
How much gravel do I need for a French drain?
Order the net trench volume: length × width × depth, minus the perforated pipe's own volume, plus a small waste allowance. For a 40 ft run of 12 in wide, 18 in deep trench with a 4 in pipe, that is roughly 2.2 cubic yards of drainage stone. Enter your dimensions and the calculator returns the volume, weight, bag count and truckloads automatically.
Should I subtract the pipe volume from the gravel?
Yes — the perforated pipe occupies space that would otherwise be stone, so its cylindrical volume (π × radius² × length) is subtracted from the trench volume to get the net gravel you order. The calculator does this for you and skips the subtraction entirely for a pipeless (gravel-only) drain.
How much landscape fabric do I need to wrap the trench?
Filter fabric area = length × (width + 2 × depth + overlap). One sheet lines the trench floor and both walls, then folds back over the top of the stone. The overlap term (6 in / 150 mm by default) covers that top fold. This is why fabric is easy to under-order — the wrap covers far more than the trench opening. Enter a roll width to also get a rolls-needed count.
What slope does a French drain need?
The widely cited minimum is about 1% — 1 inch of fall for every 8 feet of run, or 10 mm per metre. Total fall = run length × grade, so a 40 ft drain at 1% drops about 4.8 inches end to end. The calculator warns you when a segment falls below 1%. These are commonly required minimums — check local code for the figure that applies to your project.
Can I build a French drain without a pipe?
Yes. A pipeless (gravel-only) French drain is a real, recognised option for light drainage over short runs in free-draining soil. Choose 'None' for the pipe and the calculator fills the whole trench with gravel and reports no pipe length. A perforated pipe is generally preferred for clay soils, long runs, flat yards, or heavier roof and driveway runoff.
How deep and wide should the trench be?
A typical yard French drain is about 12 in wide and 18–24 in deep, with a 4 in pipe. A foundation or footing drain usually runs deeper to reach below the footing, and a shallow curtain drain intercepts surface water higher up. The profile presets seed these typical figures, but every dimension is editable — the driveway preset in particular is a typical starting point, not a standard.
What size gravel goes in a French drain?
Clean, washed, angular drainage stone — commonly 3/4 in crushed stone (#57) or pea gravel — is used so water moves freely through the voids. Because it drains by staying loose, clean drainage stone is placed without compaction, so the calculator defaults compaction to zero. Both material presets carry a typical bulk density you can edit for your supplier.
Can I use metric or imperial units?
Both. Toggle at the top of the page: metric gives metres, cubic metres and tonnes; imperial gives feet, cubic yards and short tons, with linear dimensions in inches. Switching converts every dimension and setting in place, and the calculator remembers your choice in your browser.
Is the calculator private?
Yes. Every calculation runs locally in your browser — your dimensions, materials and costs are never sent to a server. Your project is saved only in your own browser's local storage, so it is still there when you come back.