How Much Gravel for a French Drain? Tonnes by Length and Trench (UK)
A French drain is a trench lined with geotextile, holding perforated pipe in clean gravel, laid to a fall towards an outlet. A drain 15 m long in a trench 300 mm wide and 450 mm deep, with a 100 mm pipe and 10% extra gravel, takes 2.10 m³ of gravel: 3.36 tonnes at 1.6 tonnes per m³, or 5 bulk bags. The French drain calculator works out your own drain.
Gravel by drain length
Trench 300 × 450 mm, 100 mm pipe, 10% extra, 20 mm gravel at 1.6 tonnes per m³, falling 1 in 100.
| Length | Gravel (m³) | Gravel (tonnes) | Bulk bags (0.8 t) | Membrane (m²) | Soil dug out (m³) | Total fall (mm) |
|---|---|---|---|---|---|---|
| 5 m | 0.70 | 1.12 | 2 | 7.5 | 0.68 | 50 |
| 10 m | 1.40 | 2.24 | 3 | 15.0 | 1.35 | 100 |
| 15 m | 2.10 | 3.36 | 5 | 22.5 | 2.03 | 150 |
| 20 m | 2.80 | 4.48 | 6 | 30.0 | 2.70 | 200 |
| 30 m | 4.20 | 6.71 | 9 | 45.0 | 4.05 | 300 |
Gravel by trench size
For a drain 15 m long. A wider or deeper trench takes much more gravel, because the volume grows with both.
| Trench | Gravel (m³) | Gravel (tonnes) | Bulk bags (0.8 t) | Membrane (m²) | Soil dug out (m³) |
|---|---|---|---|---|---|
| 200 × 300 mm | 0.00 | 0.00 | 0 | 4.5 | 0.00 |
| 300 × 450 mm | 0.00 | 0.00 | 0 | 4.5 | 0.00 |
| 300 × 600 mm | 0.00 | 0.00 | 0 | 4.5 | 0.00 |
| 450 × 600 mm | 0.00 | 0.00 | 0 | 4.5 | 0.00 |
The fall along the trench
The trench must fall steadily to the outlet. For a drain 15 m long:
| Gradient | Total fall | Fall per metre |
|---|---|---|
| 1 in 60 | 250 mm | 16.7 mm |
| 1 in 80 | 188 mm | 12.5 mm |
| 1 in 100 | 150 mm | 10.0 mm |
| 1 in 150 | 100 mm | 6.7 mm |
The pipe maker's instructions give the minimum fall for their pipe. See the drain fall calculator for foul drain gradients under Building Regulations Part H.
Laying it
- Use clean, washed 20 mm stone with no fines. Dust and sand clog a drain.
- Lay the membrane in the trench with enough over the sides to fold across the top.
- Put a layer of gravel under the pipe, lay the pipe to the fall, then fill round and over it and fold the membrane over.
- Make sure the low end drains freely, and do not send water onto a neighbour's land or a public road.
What about a soakaway?
A soakaway is a different thing: a pit or chamber that lets rainwater soak away. Its size depends on the roof area, local rainfall and a soil infiltration test, as set out in BRE Digest 365. We have not built a soakaway calculator because we could not check that method against the Digest itself. If you need one near a building, ask building control or a drainage engineer.
These figures are estimates: the weight of gravel varies with the stone and its moisture, so check your supplier's figure. See the gravel calculator for other materials.
Frequently asked questions
How much gravel do I need for a French drain?
For a drain 15 m long in a trench 300 mm wide and 450 mm deep with a 100 mm pipe, about 3.36 tonnes of drainage gravel with 10% extra, which is 5 bulk bags of 0.8 tonnes.
How much membrane do I need?
The membrane lines the base and both sides and folds over the top, so its width is the trench width plus twice the depth plus a 300 mm overlap. For this trench, 22.5 m² for 15 m.
Does this size a soakaway?
No. A soakaway is sized from a soil infiltration test and local rainfall, using BRE Digest 365, which is not a job for a simple chart. A French drain carries water to an outlet.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and check the Building Regulations and any planning rules for anything structural or close to a boundary.
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