French Drain Calculator
Gravel, pipe and fabric for a French drain of a given length and trench size.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Trench volume = length × width × depth
Gravel = trench volume − pipe volume (π(d/2)² × length)
Fabric width = trench width + 2 × depth + 12-in overlap; fabric area = that × length
Run the calculation to see each step with your own numbers.
50 ft drain, 12-in trench, 2 ft deep, 4-in perforated pipe, fabric-wrapped, 10% waste.
Trench = 50 × 1 × 2 = 100 ft³ = 3.70 yd³
Pipe = π(2/12)² × 50 = 4.4 ft³ → gravel = 95.6 ft³ = 3.54 yd³ → order 3.9 yd³
Fabric = 50 × (1 + 4 + 1) = 300 ft² → order 330 ft²
- Rectangular trench of constant width and depth. Slope the trench ≥ 1% for flow — the extra depth over the run is minor and covered by waste.
- Gravel is washed, angular ¾-in drainage stone at ~1.35 t/yd³ (editable via the Crushed Stone Calculator's density note).
- Fabric wraps the trench floor and walls with a top overlap; a burrito wrap uses this amount. Sock-wrapped pipe still benefits from a trench liner in silty soil.
- Catch basins, pop-up emitters, cleanouts and the outfall are separate.
- Pipe or no pipe in a French drain?
- Traditionally a French drain is just a gravel-filled trench. Adding a perforated pipe at the bottom multiplies the flow capacity for almost no extra cost, so most modern installs include one — perforations down, sock or trench fabric to keep silt out.
- Which way do the pipe perforations face?
- Down. Water enters a French drain from the bottom of the trench and rises into the pipe, so the holes go on the underside. Wrap the pipe in a sock or line the trench with filter fabric to keep silt out.
- How deep and wide should the trench be?
- Wide enough for about 3 in of stone on each side of the pipe — 12 in is typical for a 4-in pipe — and deep enough to sit below whatever you are draining, sloped at least 1% toward the outlet.
Geometry
Rectangular-prism and cylinder volume; standard geometry. Unit conversions per NIST SP 811.
Stone density
Typical crushed stone (¾-in) ≈ 1.35–1.45 t/yd³ loose (≈1,600 kg/m³). Engineering ToolBox / aggregate producer data.
About the French Drain Calculator
The French Drain Calculator turns a drainage trench into quantities of washed stone, perforated pipe and filter fabric, from the trench length, width and depth. The stone is the trench volume minus the pipe. Set the pipe diameter to 0 for a classic pipe-free gravel trench, or leave it at 4 in for the modern version that carries far more water for almost no extra cost. Catch basins, cleanouts and the outfall are separate.
What you enter
- Drain length
- Trench width — Usually 2–3× the pipe diameter. 12 in is typical for a 4-in pipe.
- Trench depth
- Perforated pipe diameter — 4 in typical. Enter 0 for a pipe-free (French) trench.
- Wrap the trench in filter fabric?
- Waste allowance — Default 10%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Drainage gravel — Trench volume minus the pipe. Use washed ¾-in stone.
- Gravel weight — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Perforated pipe — the exact result of the calculation.
- Filter fabric — Trench floor + both walls + a 12-in overlap at the top. 0 if not wrapping.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Crushed Stone Calculator — Crushed stone by volume and weight. Shares the density question with the Gravel Calculator.
- Gravel Calculator — Gravel by volume and by delivered weight for driveways, paths and fill.
- Cubic Yard Calculator — Convert a length, width and depth into cubic yards, cubic feet or cubic metres.
- Retaining Wall Calculator — Block count and base material for a segmental retaining wall.
Part of the Drainage calculators.