How to use this calculator
Start by entering the total length of your planned French drain in feet. Measure along the path where you intend to dig the trench, following the natural slope of your yard from the wet area to the discharge point.
Set the trench width and depth in inches. The defaults of 12 inches wide and 18 inches deep work well for most residential yard drainage projects. If you are draining along a foundation, you may need a deeper trench that reaches the bottom of the footing.
Choose your pipe diameter based on the volume of water you need to handle. Four inch pipe is the standard for most residential French drains and handles moderate water flow from lawn and garden areas. Six inch pipe is better for properties with heavy water volume, steep slopes, or long runs over 100 feet.
Toggle catch basins on if you want to collect surface water from downspouts or low spots along the drain route. The calculator automatically suggests one catch basin per 50 feet, or you can enter your own count. Results update instantly as you adjust any input.
How a French drain works
A French drain is a gravel filled trench with a perforated pipe at the bottom. Groundwater seeps through the soil and into the gravel, which acts as a collection channel. The water then flows down through the gravel and enters the perforated pipe through small holes along its length.
Once inside the pipe, the water follows the slope of the trench to the discharge point. This can be a daylight outlet on a hillside, a pop up emitter in the lawn, a dry well, or a connection to the storm drainage system. The pipe carries water away much faster than gravel alone because it provides a smooth, unobstructed path.
Filter fabric lines the trench to keep fine soil particles from migrating into the gravel and eventually clogging it. Without fabric, silt and clay slowly fill the voids between the stones and the drain loses its ability to move water. The fabric acts as a barrier that lets water pass through while holding back the dirt.
Choosing between 4 inch and 6 inch pipe
Four inch perforated pipe is the standard choice for most residential French drains. It handles the water volume from typical lawn drainage, downspout connections, and foundation waterproofing on lots up to about half an acre. Four inch pipe costs roughly half as much as six inch, is lighter and easier to work with, and fits comfortably in a 12 inch wide trench with gravel on all sides.
Six inch pipe moves approximately twice the volume of water as four inch pipe at the same slope. Choose six inch when your drain will handle heavy water flow from large roof areas, steep slopes that funnel surface runoff, or properties with high water tables. If your drain run is longer than 100 feet, six inch pipe also helps maintain flow because it is less affected by minor sediment buildup over time. Keep in mind that six inch pipe requires a wider trench, at least 14 to 16 inches, which means more digging and more gravel.
Frequently asked questions
How deep should a French drain be?
Most residential French drains are 18 to 24 inches deep. The trench needs to be deep enough to sit below the water table or the problem area you are trying to drain. For foundation drains, the bottom of the trench should be at or slightly below the bottom of the footing. For yard drainage, 18 inches is usually sufficient to intercept surface water and shallow groundwater.
What slope does a French drain need?
A French drain needs a minimum slope of 1 percent, which equals about 1 inch of drop for every 8 feet of horizontal run. A steeper slope of 2 percent moves water faster and reduces the chance of sediment building up in the pipe. If your yard is very flat, you may need to dig the trench progressively deeper to create enough grade for the water to flow.
How long does a French drain last?
A properly installed French drain with filter fabric and clean washed gravel can last 30 to 40 years or more. The most common failure point is clogging from fine soil particles that work through the fabric and fill the gaps between the gravel. Using a filter sock on the pipe and clean washed stone (not crusher run) significantly extends the lifespan by keeping sediment out of the system.
How much does a French drain cost DIY versus professional?
DIY French drain materials typically cost $3 to $10 per linear foot depending on trench size and pipe diameter. A 50 foot drain might run $150 to $500 in materials. Professional installation for the same drain costs $25 to $70 per linear foot including labor, bringing the total to $1,250 to $3,500. The biggest DIY savings come from avoiding the labor cost of trenching, which is the most time consuming part of the job.
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This calculator provides material estimates for planning purposes only. Actual quantities may vary based on soil conditions, trench dimensions in the field, and local gravel availability. French drains near foundations or property lines may require permits or engineering review. Always check local building codes and utility locations before digging. DoubtCalc is not responsible for drainage failures, flooding, or property damage resulting from use of this calculator.