A French drain is a gravel-filled trench with a perforated pipe that collects groundwater and surface water and carries it away from foundations, lawns and driveways. Typical trenches are 6–12 inches wide and 18–24 inches deep, lined with filter fabric to stop soil clogging the gravel, with the pipe laid on a few inches of clean washed stone and a fall of at least 1% towards the outlet.
How to use the French Drain Calculator
- Drain length
- Trench width
- Trench depth
- Pipe diameter
- Pipe section length
- Gravel density
- Gravel price
- Price per pipe section
- Filter fabric price
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the trench length, width and depth, the pipe diameter and length of each pipe section, and prices. The calculator subtracts the pipe volume from the trench to find the gravel, converts it to tons with the density of washed stone and adds the fabric and pipe.
Tips for an accurate estimate
- Sketch the job and split irregular areas into rectangles, circles and triangles before you measure.
- Add a waste allowance – typically 5–10% for simple layouts and 15–20% for diagonal, curved or heavily cut work.
- Check the coverage or yield printed on the product, because sizes and densities vary between manufacturers.
- Buy all units of a finished material (tile, brick, siding) from the same batch so colours match, and keep a few spares for repairs.
- Confirm local prices – quarry, lumber and steel prices vary widely by region and season.
Common mistakes to avoid
- Ordering exactly the calculated amount with no allowance for breakage, offcuts and settlement.
- Forgetting to subtract doors, windows and other openings – or subtracting small ones that still need cutting around.
- Confusing US short tons (2,000 lb) with metric tonnes (1,000 kg) when comparing quotes.
Inputs explained
| Field | What to enter |
|---|---|
| Drain length | Units: ft, m, in, yd, cm |
| Trench width | Units: in, cm, mm, ft, m |
| Trench depth | Units: in, cm, mm, ft, m |
| Pipe diameter | Choose one: 3 in, 4 in, 6 in, 100 mm … |
| Pipe section length | Choose one: 10 ft, 20 ft, 100 ft coil, 3 m … |
| Gravel density | Units: kg/m³, lb/ft³, g/cm³, lb/yd³ |
| Gravel price | Units: /US ton, /t, /lb, /kg |
| Price per pipe section | Units: /piece |
| Filter fabric price | Units: /ft², /m², /yd² |
What each result means
- Trench volume = [Drain length]×[Trench width]×[Trench depth]
- Gravel volume = [Trench volume] – pi÷4×[Pipe diameter]^2×[Drain length]
- Gravel weight = [Gravel volume]×[Gravel density]
- Pipe sections = ceil([Drain length]÷[Pipe section length] – 0.000001)
- Filter fabric = [Drain length]×(2×[Trench depth] + 2×[Trench width])
- Material cost = [Gravel weight]×[Gravel price] + [Pipe sections]×[Price per pipe section] + [Filter fabric]×[Filter fabric price]
Example values
How the main result changes when Drain length changes and everything else stays at the example values above:
| Drain length (ft) | Gravel volume (yd³) |
|---|---|
| 25 | 1.77 |
| 37.5 | 2.66 |
| 50 | 3.54 |
| 62.5 | 4.43 |
| 75 | 5.31 |
| 100 | 7.08 |
Formula
Trench volume = L × W × DGravel = trench volume − π/4 × pipe ز × LFabric = L × (2D + 2W) (wrap with overlap)Pipe pieces = ceil(L ÷ piece length)Worked example
Inputs — Drain length: 50 ft; Trench width: 12 in; Trench depth: 24 in; Pipe diameter: 4 in; Pipe section length: 10 ft; Gravel density: 1,522 kg/m³; Gravel price: $40 /US ton; Price per pipe section: $12 /piece; Filter fabric price: $0.25 /ft².
Results — Trench volume = 3.7 yd³; Gravel volume = 3.54 yd³; Gravel weight = 4.54 US ton; Pipe sections = 5; Filter fabric = 300 ft²; Material cost = $316.74.
Frequently asked questions
How deep should a French drain be?
Usually 18–24 in deep and 9–12 in wide; deeper if you are intercepting water next to a basement wall.
What gravel is best for a French drain?
Clean washed ¾–1½ in stone (#57 or #3). Avoid stone with fines, which clogs.
Which way do the pipe holes face?
Down. Water rises into the pipe from below and the holes are less likely to fill with sediment.
How much slope does a French drain need?
At least 1% (⅛ in per foot), ideally 2% (¼ in per foot).
What formula does the French Drain Calculator use?
Trench volume = L × W × D.