U.S. construction · site drainage
French drain planner.
See fall, changing trench depth, gravel after cover and pipe displacement, fabric wrap, stock lengths, couplers, and spoil before the first shovel goes in. Calculations run locally; this tool does not transmit your form values.
Trench profile and materials
Drain profile and order
Formula, source, and limits
Method reviewed September 30, 2026: fall equals horizontal run × entered slope. Ending depth equals starting depth + ending ground elevation change + fall. Excavation uses the full changing-depth trench prism. Gravel reserves your entered top cover depth, then subtracts cylindrical pipe displacement before allowance. Fabric wraps the bottom and sides of that gravel zone plus your overlap.
Trade source: the NDS French Drain Installation Guide, accessed September 30, 2026, describes a sloped trench with perforated pipe, gravel, and landscape fabric directed to an outlet. The USDA NRCS Subsurface Drain Standard 606, July 2022, documents filter and envelope considerations; it is not used here as a residential design prescription.
Boundary: This is geometry and purchasing math, not hydraulic sizing, code review, outlet approval, excavation safety, or engineering. Confirm utilities before digging and verify soil, groundwater, discharge rights, permits, and local drainage requirements.
Frequently asked questions
Does the planner choose a slope?
No. You enter the project slope; the page shows the resulting fall and changing depth without claiming a universal value.
Are top cover and pipe displacement removed from gravel?
Yes. Enter the topsoil, sod, or other cover depth you intend to reserve. The calculator then subtracts the sloped pipe cylinder from the remaining gravel zone before applying the allowance.
Does this approve the outlet?
No. The outlet and its effects are site and jurisdiction decisions. Confirm them before ordering or excavation.