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French Drain Calculator (UK): Gravel, Pipe and Fabric in Metres

Enter the length of the drain and the size of the trench. You get the tonnes and cubic metres of drainage gravel, the bulk bags, the perforated land drain pipe, the geotextile membrane, the soil you dig out and the fall along the trench.

UK edition. Looking for the US equivalent (feet, inches and US gallons)? Use the US version.

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Your project

300 mm is a common width for a garden land drain. A pipe of 100 mm needs at least about 200 mm.

Outside diameters differ a little between makers: read the label.

Steeper is safer. Follow the pipe maker for the minimum.

%

For uneven trench walls and settling.

tonnes

Most bulk bags hold about 0.8 to 0.85 tonnes.

Results

Example job shown. Change the numbers to match yours.

Drainage gravel
3.36 tonnes
Clean 20 mm stone at 1.6 tonnes per m³, less the pipe, with your extra
Gravel, cubic metres
2.10 m³
Bulk bags of gravel0.80 tonnes each
5 bags

Pipe and membrane

Perforated pipePlus couplers; sold in coils or lengths
15 m
Geotextile membraneLines the trench and wraps over the gravel, with a 300 mm overlap
22.5 m²

Digging

Soil to dig outMeasured in the ground: it bulks up once dug
2.03 m³
Total fall along the trenchThe length ÷ the gradient
150 mm
  • A French drain is a trench lined with geotextile, with perforated pipe bedded in clean gravel, laid to a fall towards an outlet such as a ditch, a soakaway or a surface water drain. Without a fall and an outlet it simply fills with water. This calculator sizes the trench from its geometry only. It does not size a soakaway or say how much water the ground can take, which depends on a soil infiltration test (BRE Digest 365) and on Building Regulations Part H. For anything near a building, ask your building control office.

Before you buy: round up to whole packs, check the size printed on the label, and add a little spare. Your supplier has the final say.

How this calculator works

The gravel fills the trench around the pipe. Multiply the length, width and depth of the trench to get its volume in cubic metres, subtract the volume of the pipe, add your extra, and multiply by the weight of a cubic metre of drainage gravel, about 1.6 tonnes.

gravel (tonnes) = (length × width × depth − pipe volume) × (1 + extra %) × 1.6

Worked example: a drain 15 m long in a trench 0.3 m wide and 0.45 m deep has 15 × 0.3 × 0.45 = 2.025 m³ of trench. A 100 mm pipe takes up 0.118 m³, leaving 1.907 m³, or 2.10 m³ with 10% extra: 3.36 tonnes of gravel, which is 5 bulk bags of 0.8 tonnes. You also need 15 m of pipe and 22.5 m² of membrane (15 × (0.3 + 2 × 0.45 + 0.3)), and you dig out 2.03 m³. At 1 in 100 the trench falls 150 mm over its length. A 30 m drain takes 6.71 tonnes, and a wider trench of 450 × 600 mm over 15 m takes 0.00 tonnes.

What this does not do: it does not size a soakaway. The method in BRE Digest 365 depends on local rainfall and on a soil infiltration test, so it is not something a simple calculator can safely do. Ask your building control office or a drainage engineer.

Assumptions and sources

The assumptions behind your numbers (waste, sizes, rates) are listed in the notes directly under your results, and they update when you change an input.

The formula and a worked example are shown above, so you can check any result by hand. Read how we calculate and check our numbers. Results are estimates for planning: supplier products vary, so check the size printed on the label.

Gravel by drain length (300 × 450 mm trench, 100 mm pipe, 10% extra)

Clean 20 mm drainage gravel at 1.6 tonnes per m³.

LengthGravelBulk bagsMembraneTotal fall at 1 in 100
5 m1.12 tonnes27.5 m²50 mm
10 m2.24 tonnes315.0 m²100 mm
15 m3.36 tonnes522.5 m²150 mm
20 m4.48 tonnes630.0 m²200 mm
30 m6.71 tonnes945.0 m²300 mm

Tips before you buy

  • Use clean, washed, angular stone of about 20 mm, with no fines. Fines (dust and sand) clog the drain. Crusher run and MOT Type 1 are not suitable.
  • Line the trench with a non-woven geotextile and fold it over the top of the gravel, so soil cannot wash in.
  • Check the fall with a string line and a level, and make sure there is a free-draining outlet at the low end before you dig.
  • Do not send water onto a neighbour's land or onto a public highway. A soakaway near a building needs a proper infiltration test: ask building control.
  • Check plans and use a cable avoidance tool for pipes and cables before you dig a trench.

Tools & materials for this project

Affiliate links As an Amazon Associate we earn from qualifying purchases. These links do not change what you pay.

Perforated land drain pipeGeotextile membraneDrainage gravel (bulk bag)Land drain couplersTrenching spade

Frequently asked questions

How much gravel do I need for a French drain?

Multiply length × width × depth in metres, subtract the pipe, add 10%, and multiply by 1.6 for tonnes. A 15 m drain in a 300 × 450 mm trench takes about 3.36 tonnes (2.10 m³).

How deep should a French drain be?

Often 450 to 600 mm for a garden. It needs to be deep enough to collect the water and to fall to the outlet, so check your levels before you dig.

What fall does a French drain need?

A steady fall towards the outlet. 1 in 100 is 10 mm in every metre. Steeper is safer, and the pipe maker's instructions are the guide.

Is this the same as a soakaway?

No. A soakaway is a pit or chamber that lets water soak into the ground, and is sized from a soil infiltration test (BRE Digest 365). A French drain carries water along to an outlet. This calculator does not size a soakaway.

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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.