Free french drain calculator: excavation, stone, geotextile and pipe
Get the excavation, the drain stone, the geotextile and the pipe of a french drain in seconds, free and without an account. One row per straight stretch of drain: its length, the trench's width and depth, and the stone layer under, around and over the pipe. Add the slope you give the pipe: the calculation gives the excavation, the stone, the geotextile that wraps it, the pipe length and the drop over the longest stretch. In imperial or metric.
French drains: what the sources say (reference)
The rules and products our template cites, as read. Your municipality and your local code have the last word.
| What the sources say | |
|---|---|
| Stone | Quebec guide: clean 50 mm (2 in) stone, wrapped in geotextile |
| Pipe | ADS: 4 in (100 mm) single wall perforated pipe, in 10, 50, 100 (30.5 m) and 250 ft (76.2 m) coils |
| Grade | IRC: the grade falls at least 6 in within the first 10 ft from the house. NBC: backfill graded so water drains away from the foundation |
| Outlet | Quebec guide: consult the municipality before sending yard or foundation water anywhere. Oregon State: never onto a neighbor without an easement |
| Before digging | A free locate: 811 in the US, Info-Excavation in Quebec (3 business days ahead) |
Sources: Quebec stormwater guide; ADS, product catalog; IRC 2021, R401.3; NBC 2020, 9.12.4.1; Oregon State Extension, drywells; 811 (US); Info-Excavation
How it is calculated
Excavation = length x width / 12 x depth / 12 / 27 cu yd, per stretch, added up. Drain stone = length x width / 12 x stone layer / 12 / 27 cu yd. Geotextile = length x 2 x (width + stone layer) / 12 sq ft: both sides, the top and the bottom of the stone layer, with no overlap. Drain length = the stretches added up, the perforated pipe to lay. Drop = length x 12 x slope / 100 in, for the longest stretch: 1% drops 1.2 in every 10 ft. The same formulas as the yard drainage survey template. Every size and the slope are yours: nothing is prefilled.
Example: a soggy back yard
From our template page's demo: two stretches of trench 12 in wide, 60 ft along the back fence, 24 in deep with 18 in of stone, and 25 ft by the shed, 18 in deep with 12 in of stone, at a 1% slope. That makes 85 ft of drain, 5.83 cu yd to dig, 4.26 cu yd of stone and 400 sq ft of geotextile, and the longer stretch drops 7.2 in to its outlet.
The whole job in MesuPro
Yard drainage survey
French drains and dry wells: trench, stone, geotextile, water to hold, pit volume and footprint, hours to empty.
You get: Water to hold
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