A French drain is a trench of washed gravel around a perforated pipe, wrapped in fabric, that collects water from soggy ground and carries it to a lower outlet. The calculator sizes one straight run: the gravel in cubic yards and tons, the pipe, the fabric that wraps the gravel, the soil left to cover it, and how much the trench must fall from end to end.
The defaults are the ones public agencies publish. The Northern Virginia Soil and Water Conservation District describes a standard French drain ditch as about 1.5 feet deep and 10 to 12 inches wide, with a pipe usually 4 to 6 inches across. The Department of Energy's guidance on footing drains puts at least 2 inches of washed gravel under the pipe and 6 over it, and Washington State University Extension slopes the trench between 0.5 and 1 percent.
How to use it
- 01
Find the outlet first
A French drain moves water; it does not make it disappear. Delaware's environmental agency says it routes water to a natural drainage site such as a stream, swale, ditch or slope, and that the drain must not cause flooding on a neighbor's property. Wherever the pipe ends, that outlet has to sit lower than the wet spot, so walk the route with a level before you dig.
- 02
Measure the run and set the trench
Enter the trench's length in feet, then its width and depth in inches. Start from the district's 12 by 18 inch ditch; Delaware calls simple drains 8 to 24 inches deep a do-it-yourself job, and deeper ones, or ones near the foundation, work for a drainage contractor. Call 811 a few days before any digging.
- 03
Pick the pipe
A 4-inch perforated pipe is the common size; a 6-inch pipe carries more water but needs a deeper gravel layer, 14 inches instead of 12. If the trench is too shallow for the gravel, the calculator says so.
- 04
Price it at your supplier's rates
Enter the price of a ton of gravel, a foot of pipe and a square foot of fabric. No price is filled in.
How the math works
Gravel
The gravel fills the trench's width to a depth of 2 inches under the pipe, the pipe itself, and 6 inches over it, less the pipe's own volume at its nominal diameter. For a 50-foot trench 12 inches wide with a 4-inch pipe, that is 12 inches of gravel: 50 cubic feet less 4.4 for the pipe, about 1.69 cubic yards.
Gravel by weight
Tons = cubic yards × 1.34, the Tennessee Department of Transportation's factor for No. 57 stone that has not been compacted, which is how drain gravel goes in. The Department of Energy asks for washed ½ to ¾ inch gravel; No. 57 stone runs from 1 inch down, so ask the supplier what its drain rock weighs and use that. The 50-foot trench takes about 2.3 tons.
Fabric and pipe
The fabric goes under, around and over the gravel, so its width is the gravel's outline: twice the trench width plus twice the gravel's depth, 48 inches for the 50-foot trench, 200 square feet in all. Add the overlap the fabric's maker asks for at the top; the calculator adds none. The pipe runs the trench's length.
Fall
At 0.5 percent the trench drops 0.6 inch for every 10 feet, at 1 percent 1.2 inches: 3 to 6 inches over 50 feet. The calculator gives the fall for your length; set the bottom of the trench to it with a string line and a level.
Worked examples
| Length | Pipe | Gravel | Tons | Fabric |
|---|---|---|---|---|
| 25 ft | 4 in | 0.85 cu yd | 1.1 | 100 sq ft |
| 50 ft | 4 in | 1.69 cu yd | 2.3 | 200 sq ft |
| 100 ft | 4 in | 3.38 cu yd | 4.5 | 400 sq ft |
| 50 ft | 6 in | 1.80 cu yd | 2.4 | 217 sq ft |
Tons use TDOT's factor for uncompacted No. 57 stone; fabric is before any overlap.
Where the number goes wrong
- Digging before finding a lower outlet. With nowhere lower to go, the trench fills and stays full.
- Using gravel with fines in it. The Department of Energy warns that silt and sand in the gravel clog the pipe's perforations.
- Skipping the fabric over the top. Washington State University Extension wraps it over the gravel so soil cannot fill the spaces between the stones.
- Running the drain toward a neighbor's lot. Delaware's guide says the drain must not cause flooding on neighboring property.
- Building it where the ground stays saturated. WSU says French drains work best above the groundwater table.
Questions people ask
How deep and wide should a French drain be?
The Northern Virginia Soil and Water Conservation District puts a standard French drain at about 1.5 feet deep and 10 to 12 inches wide. Delaware's guide calls 8 to 24 inches deep a simple do-it-yourself drain; deeper ones are a contractor's job.
How much gravel do I need for a French drain?
For a 50-foot trench 12 inches wide with a 4-inch pipe and the Department of Energy's 2 inches of gravel under the pipe and 6 over it, about 1.69 cubic yards, or 2.3 tons of uncompacted No. 57 stone.
How much slope does a French drain need?
Washington State University Extension says 0.5 to 1 percent: a drop of 1 foot over 100 feet is 1 percent. Over 50 feet that is 3 to 6 inches.
Do I need a permit for a French drain?
Delaware's guide sends homeowners to their city or county for building permits and local flood rules. Grading, work in a floodplain and anything that discharges to a street or a storm drain are the usual triggers; the grading permit guide covers the first.