French Drain Calculator
Find out how much gravel, how many pipes, and optional filter fabric you need for any French drain project using this French drain calculator.
Volume to order2.6162 cubic yards70.637 cu ft · 2.0002 m³
Weight3.7042 US tons3,360.3 kg · 7,408.3 lb · 3.3603 tonnes
MaterialGravel (loose)1,680 kg/m³, about 105 lb/ft³
Cubic feet70.637
Cubic metres2.0002
In 50 lb bags149if buying bagged rather than bulk
Trench volume2.7778 yd³75 cu ft excavated
Pipe displaces4.3633 cu ft4 in pipe over 50 ft
Share of the trench the pipe fills5.82%
Trench cross-section216 sq in12 × 18 in
Spoil to remove3.472 yd³excavated soil bulks up by roughly 25%
The method
gravel = trench volume − pipe volume
The pipe takes up room
A 4-inch pipe running the length of a 12 × 18 inch trench displaces about 9% of it, so deducting the pipe matters on a long run. Wrap the gravel in filter fabric and the drain keeps working; leave it out and silt fills the voids within a few years.
Excavated soil does not go back in the hole it came from. Digging breaks up the structure and it bulks up by 20–30%, so plan somewhere for the surplus.