Estimate french drain gravel fill, perforated pipe, filter fabric, excavation volume, and slope drop from trench geometry, pipe size, and drainage run. Use it to test different inputs quickly, compare outcomes, and understand the main factors behind the result before moving on to related tools or deeper guidance.
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French drain trench planner Estimate gravel fill, perforated pipe, filter fabric, excavation volume, and total slope drop from trench geometry, pipe size, and drainage run.
Common perforated pipe sizes
Enter trench and slope details Add the trench length, width, depth, perforated pipe size, and slope to estimate gravel fill, fabric coverage, excavation volume, and drain fall.
French drain gravel, pipe, fabric, and trench-fall planning
A French drain calculator turns trench geometry into a practical material list before you start digging. It estimates excavation volume, gravel fill, perforated pipe length, filter-fabric coverage, and total fall from the selected slope so you can compare the trench concept with the space available on site.
What this French drain calculator is estimating
A practical French drain estimate is more than just trench length. The material total depends on trench width, trench depth, perforated pipe diameter, the amount of filter fabric needed to wrap the aggregate, and the fall required to move water toward the outlet.
This calculator treats the trench as a simple rectangular excavation with a round perforated pipe. It subtracts the pipe displacement from the gravel fill so the aggregate total is closer to the material you actually need to order.
Core trench and gravel formulas
The excavation volume starts with trench length multiplied by trench width and trench depth. Gravel volume then removes the pipe cylinder from that trench volume before any waste allowance is added for trimming, spillage, and practical ordering.
Filter fabric coverage is estimated from trench width plus both trench walls plus the selected overlap. That gives you a rough wrap width multiplied by trench length, which is more useful for early roll planning than area alone.
Why slope matters
A French drain only works if the trench falls consistently toward its discharge point. The calculator converts the selected slope percentage into total fall over the entered trench length so you can see the drop you need to maintain in the field.
For example, a 100 ft run at 1 percent slope needs 1 ft of fall from start to finish. That is simple to say, but it is easy to miss once the trench starts turning, crossing grades, or tying into an outlet at a constrained elevation.
Worked example
A 100 ft trench that is 12 inches wide and 18 inches deep with 4 inch perforated pipe needs about 5.56 cubic yards of excavation. After pipe displacement and a 10 percent material allowance, the gravel order is about 5.76 cubic yards, with about 110 ft of pipe and roughly 550 sq ft of filter fabric.
That same run at 1 percent slope needs 1 ft of fall from inlet to outlet. If the available site grades cannot deliver that fall cleanly, the trench layout or outlet point should be rechecked before excavation starts.
Frequently asked questions
How much gravel goes in a French drain?
That depends on trench length, width, depth, pipe size, and whether you add a waste allowance. This calculator estimates gravel by taking the trench volume and subtracting the displaced pipe volume before adding any ordering allowance.
What slope should a French drain have?
Many installers aim for at least about 1 percent fall, though the exact target depends on the pipe type, outlet conditions, and the site layout. The key is consistent downhill fall toward the discharge point rather than isolated low spots.
Why does a French drain need filter fabric?
Filter fabric helps separate the free-draining aggregate from surrounding soil so the stone and perforated pipe do not clog as quickly with fines. The exact fabric specification should still match the soil and drainage design.
Does this calculator size the whole drainage system?
No. It is a trench and material planner. Outlet design, water loading, local code, soil conditions, and final pipe selection still need site-specific review.
