French Drain Calculator

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The run of the drain from the catch point to the outlet.
A residential French drain trench is usually 20-45 cm (8-18 in) wide.
Typically 45-60 cm (18-24 in) so the pipe sits below the water you want to drain.
A 100 mm (4 in) perforated pipe is the standard for most home drainage.
Used to work out how many lengths of pipe to buy.
Minimum 1% (1 cm per metre) keeps water moving to the outlet.
%
Extra for compaction, spillage and uneven trench walls. 10% is typical.
%
Loose drainage gravel is roughly 1.4-1.7 t/m³; 1.5 t/m³ is a safe default.
t/m³
Currency
Gravel to order (with waste)
2.09t
Trench (dig) volume1.35m³
Gravel volume (net)1.269m³
Pipe displaces0.081m³
Gravel (cubic yards, with waste)1.83yd³
Perforated pipe10m
Pipe lengths to buy4
Filter fabric15m²
Total fall over the run10cm

Order about 2.09 tonnes of gravel for this drain.

  • Dig about 1.35 m³ of trench and backfill with ~2.09 tonnes of washed drainage gravel (the waste allowance is already included).
  • Buy 4 lengths of perforated pipe to cover the 10.0 m run, plus a fitting for the outlet.
  • Line the trench with about 15.0 m² of geotextile filter fabric so silt cannot clog the gravel.

Next stepKeep the trench falling at least 1% (you have 10.0 cm of drop here) toward a lower outlet or soakaway.

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