Loan French Drain Estimator
A French drain is a gravel trench with a pipe hiding in it — and the gravel is always more than the trench "looks like", minus a pipe's worth everyone forgets to subtract. Enter the run and trench size: get gravel yards and tons, pipe and fabric.
Sized for the standard 4-in perforated pipe. Slope matters more than size: the trench needs at least 1% fall (about ⅛ in per foot) toward daylight or a dry well, or you have built a very long, gravel-filled puddle.
How it works
Gravel fills the trench minus the pipe: trench volume (length × width × depth) minus the 4-in pipe's cylinder volume — on a 50-ft run the pipe displaces about 4.4 cu ft, small but real. Washed drainage stone converts at 1.40 tons per cubic yard — round, washed rock, not crushed base (crushed fines clog drains; that is why drainage calls for washed stone). Pipe is bought to the trench length; add your outlet run to daylight when you order. Fabric lines the trench before stone: order the run length in a roll wide enough to wrap the trench (about 5 ft of wrap width for a standard trench).
Formula & worked example
Formula: Gravel volume = trench volume (length × width × depth) − pipe volume (π × (2 in radius in ft)² × length for standard 4-in pipe). Washed drainage stone converts at 1.40 tons per cu yd. Pipe and fabric order to the trench length.
Worked example: A 50-ft trench, 12 in wide × 18 in deep: 2.62 cu yd (3.66 tons) of washed stone, 50 ft of pipe, 50 ft of fabric.
When to use this
- A yard corner stays swampy and you are trenching a drain to daylight.
- You are ordering washed stone and need pipe and fabric on the same list.
- A drainage quote lists gravel tonnage you want to check.
How to use it, step by step
- Measure the trench run in feet along its actual path.
- Pick width and depth for the water problem — 12 × 18 in standard.
- Note gravel yards AND tons; yards order the truck, tons verify it.
- Add your outlet run beyond the trench when ordering pipe.
What the inputs mean
| Input | What it is | Typical values |
|---|---|---|
| Trench length | The full drain run | 30–100 ft typical yard drains |
| Trench width | Excavated width | 9 in light; 12 in standard; 18 in wet sites |
| Trench depth | Invert depth below grade | 12 in surface water; 18–24 in standard |
Another worked example
A short side-yard drain: 30 ft at 12 in wide and 12 in deep. Trench volume 30 cu ft; the 4-in pipe displaces 2.62 cu ft; gravel = 27.38 cu ft = 1.01 cu yd, about 1.42 tons — a single bulk scoop at most yards, plus 30 ft each of pipe and fabric.
Frequently asked questions
How much gravel for a 50-ft French drain?
A standard 12-in wide, 18-in deep trench: 2.62 cu yd of washed stone — 3.66 tons — after the 4-in pipe's own volume comes out. Plus 50 ft of perforated pipe and a 50-ft run of filter fabric.
How deep should a French drain be?
Twelve to 24 inches covers most yard work: deep enough to intercept water moving through topsoil, shallow enough to daylight at a slope. A 30-ft run at 12-in depth takes about 1.01 cu yd (1.42 tons). Foundation drains sit at footing depth — a different, bigger job.
Washed stone or crushed gravel?
Washed, round-to-subangular drainage stone (often sold as #57 or drain rock). Crushed stone carries fines that migrate into the pipe slots and silt the system shut. The fabric wrap slows that further — it is a filter, not a liner.
Does the pipe go holes-up or holes-down?
Perforated pipe in a French drain goes holes DOWN: water rises into the pipe from below as the trench fills, then flows along it. (Holes-up is for distribution systems like septic fields — different job, opposite logic.)
How much fall does the trench need?
One percent minimum — about 1 inch per 8 feet. More is better; dead level is a storage trench that eventually silts. Laser levels earn their keep on long runs.
🧱 Draining behind a retaining wall too?
Retaining walls live or die on drainage: the Retaining Wall Estimator counts the backfill stone and base gravel that keep a wall standing straight.
Can a French drain handle roof water?
It can, but solid (non-perforated) pipe is the right carrier for downspout water to the drain field — mixing roof volume into a perforated yard drain can overload it and surface water exactly where you buried the pipe.
How long does a French drain last?
A fabric-wrapped drain in washed stone commonly serves 20–40 years; unwrapped drains in fine soils can silt in under a decade. The fabric and stone choices in this estimator are the longevity choices.