French Drain Cost Calculator
Estimate the cost of an exterior French drain from your trench length and price per foot, and get the gravel cubic yards and perforated pipe the trench needs.
Calculator
A 60 ft exterior French drain at $30.00/ft is about $1,800.00, and the trench holds ≈ 3.33 cu yd of gravel. Trenching near a foundation is excavation — use licensed pros and pull a permit.
A French drain is a gravel-filled trench with a perforated pipe at the bottom that collects water and carries it, by gravity, away from where you do not want it — typically away from a foundation, a soggy yard or a retaining wall. The cost is driven almost entirely by two things you can measure yourself: how long the trench is and how much your installer charges per foot. This calculator keeps those two numbers front and center, then adds the material take-off (gravel and pipe) so you can sanity-check what a contractor lists on a quote.
Because the price per foot varies enormously by region, access, depth and whether the work is machine-dug or hand-dug, the tool never assumes a price for you. You enter the $/linear foot from your own quote; the math is the same forever regardless of what materials or labor cost this year. The gravel volume uses the standard cubic-yard conversion (length × width × depth ÷ 27), the same arithmetic a supplier uses when you order stone.
Formula
The tool runs three independent take-offs from your trench dimensions:
- Install cost = trench length (ft) × your $/linear ft
- Gravel volume = length × width × depth ÷ 27 → cubic yards
- Perforated pipe = trench length (linear ft)
Width and depth default to a typical exterior trench — about 1 ft (12 in) wide and 1.5 ft (18 in) deep — but you should override them with your own dig. The ÷ 27 converts cubic feet to cubic yards, because a cubic yard is 27 cubic feet; that is why you enter width and depth in feet. Gravel is usually ordered by the cubic yard, so round the result up when you place the order.
Worked example
Say you are draining a chronically wet side yard with a 60 ft trench, your contractor quotes $30 per linear foot installed, and the trench is 1 ft wide and 1.5 ft deep:
- Install → 60 × $30 = $1,800
- Gravel → 60 × 1 × 1.5 ÷ 27 = 3.333 cu yd (round up to about 4 cu yd ordered)
- Pipe → 60 linear ft of perforated pipe
Change any figure and the take-off updates. A deeper 2 ft trench, for example, lifts the gravel to 60 × 1 × 2 ÷ 27 = 4.44 cu yd, while the install cost tracks the price per foot you were quoted.
Background & practice
An exterior (or “curtain”) French drain that runs alongside a foundation is real excavation: the trench is dug close to the footing, often several feet deep, and back-filled with washed gravel over a perforated, sock-wrapped pipe. That places it firmly in the estimate-plus-permit category above — digging next to a foundation can undermine it if done carelessly, and many jurisdictions require a permit for excavation near a structure. Have a licensed structural or geotechnical engineer assess a foundation-water problem before you commit to a trench, and use licensed pros for the dig.
What the price per foot usually covers
When you pull the $/ft off a quote, check what it includes. A typical installed foot bundles excavation, washed gravel, perforated pipe, filter fabric (the “sock” that keeps silt out), back-fill and haul-away of spoil. Extras that are usually not in the per-foot number — and that you should add as separate lines — include a connection to a sump or daylight outlet, a pop-up emitter, catch basins, hardscape removal (a patio or driveway you have to cut through) and restoration of the lawn. If your quote itemizes those separately, keep them separate here rather than inflating the per-foot figure.
Slope, gravel and pipe conventions
Drain tile is normally laid to a slope of roughly 1% (about 1 in of fall every 8 ft) so water keeps moving. The gravel envelope surrounds the pipe on all sides; contractors commonly use washed 3/4 in stone. The pipe length equals the trench length in this tool, but remember to add a bit for the run to the outlet or sump. These are planning conventions, not code — confirm depth, slope and outlet with your installer and local building department.
Interior vs exterior
The trench outside the house is different work from an interior drain tile system, which sits inside the footing under the basement slab and routes water to a sump. If you are pricing the inside version, use the interior drain tile calculator instead; the two are often compared side by side when a basement leaks, which is why they are cross-linked here and in the guides.
Frequently asked questions
How much does a French drain cost per foot?
It varies widely by region, depth, access and whether the dig is by machine or by hand, so this tool does not assume a figure — you enter the $/linear foot from your own quote. A 60 ft trench at $30/ft works out to $1,800 in the worked example; plug in your own numbers to price your run.
How much gravel do I need for a French drain?
Gravel volume = length × width × depth ÷ 27 cubic yards. A 60 ft trench that is 1 ft wide and 1.5 ft deep needs 60 × 1 × 1.5 ÷ 27 = 3.333 cubic yards — round up to about 4 cu yd when you order, since stone is sold by the yard.
Does a French drain need a permit?
Often, yes — especially an exterior drain dug near a foundation, which counts as excavation. Confirm scope, permits and code with your local building department, and have a licensed engineer assess a foundation-water problem before you trench.
What is the difference between an exterior French drain and interior drain tile?
An exterior French drain is a gravel trench outside the house; interior drain tile is pipe laid inside the footing under the slab that feeds a sump pump. They solve overlapping problems in different ways — use the interior drain tile calculator for the inside version.
How much fall (slope) should the pipe have?
A typical convention is about 1% — roughly 1 inch of drop every 8 feet — so water keeps flowing to the outlet or sump. Treat that as a planning band and confirm the actual grade with your installer.