What is a French drain?
A French drain is a gravel filled trench with a perforated pipe at the bottom. Water soaks into the gravel, runs into the pipe and flows away to a lower point, a soakaway or a storm drain. It is the standard fix for a soggy lawn or water pooling against a wall.
Planning the rest of the job? Use the Drainage calculator and the Landscape fabric calculator too.
What are typical trench sizes?
Lawn or garden drain
12 in wide, 18 in deep (30 × 45 cm)
Along a foundation
12 to 18 in wide, 24 in deep (30 to 45 × 60 cm)
Pipe size
4 in (100 mm) for most homes
Fall
At least 1 in per 8 ft (1%)
How does the calculation work?
Gravel = trench volume − pipe volume
A drain 50 ft long, 12 in wide and 18 in deep holds 75 ft³. Take away about 4 ft³ for a 4 in pipe and you need 71 ft³, or 2.6 yd³, roughly 3.5 tons of washed gravel.
Fabric lines the trench and folds over the top, so its width is two depths plus two widths plus about 1 ft (30 cm) of overlap.