French Drain Calculator
Estimate French drain gravel, pipe length, filter fabric, trench volume, slope drop, bags, tons, and optional DIY material cost.
This is a material planning calculator, not a drainage design approval tool. For a simple rock bed without pipe, fabric, or slope planning, use the Gravel Calculator.
Results
Planning estimate only. French drain performance depends on soil, slope, outlet location, pipe, filter fabric, frost depth, site grading, and local requirements. This calculator estimates materials, not drainage approval or hydraulic capacity.
How to use this calculator
- 01Choose Imperial or Metric units.
- 02Enter the drain length, trench width, and trench depth.
- 03Enter the perforated pipe diameter, or use 0 for a gravel-only trench.
- 04Choose a drainage stone density preset or enter your supplier's density.
- 05Set waste and slope so the calculator can estimate extra gravel and total fall over the run.
- 06Include filter fabric when the trench detail calls for a fabric wrap, then enter the overlap.
- 07Use Gravel Needed, Gravel Weight, Pipe Length, Total Drop, and Filter Fabric Area as planning takeoff quantities.
- 08Open Add cost estimate only when you want a rough DIY material cost for gravel, pipe, fabric, and allowances.
Understanding the math
French drain material takeoff starts with the trench. The calculator finds trench volume, subtracts the round pipe volume, adds waste to gravel, then estimates pipe length, fabric wrap, and slope drop.
trench_volume = drain_length x trench_width x trench_depth pipe_volume = pi x (pipe_diameter / 2)^2 x drain_length
gravel_before_waste = trench_volume - pipe_volume gravel_needed = gravel_before_waste x (1 + waste_percent / 100)
total_drop = drain_length x (slope_percent / 100) fabric_area = drain_length x (trench_width + 2 x trench_depth + overlap)
Cost inputs are optional and material-only. Drain outlet, cleanouts, soil conditions, excavation, utility conflicts, and local stormwater rules can change the final design.
French drain reference table
Common planning values for French drain takeoffs. Use the active unit system, then confirm final dimensions with the project detail, supplier data, and site conditions.
| Item | Planning value | Note |
|---|---|---|
| Trench depth | 18-36 in | Depends on site, outlet, and cover needs |
| Trench width | 9-18 in | Wider trenches need more gravel and fabric |
| Perforated pipe | 4 in | Common residential French drain pipe size |
| Drainage stone | 1.35-1.50 tons/yd³ | Supplier density should override defaults |
| Slope | 0.5-1% | Confirm outlet elevation and site requirements |
| Fabric overlap | 6-12 in | Overlap helps wrap gravel and reduce soil migration |
| Outlet | Daylight or approved discharge | Avoid sending water where it creates a new problem |
Frequently asked questions
How do you calculate a French drain?
Start with drain length, trench width, and trench depth to get trench volume. Subtract the round pipe volume, add waste for gravel, then estimate pipe length, fabric area, and slope drop from the same layout.
How much gravel do I need for a French drain?
Gravel depends on trench volume, pipe diameter, gravel density, and waste. This calculator subtracts the pipe space from the trench, adds waste, then reports gravel volume and weight for ordering.
What slope should a French drain have?
Many French drain layouts use a small positive slope such as 0.5% to 1%, but the right slope depends on outlet elevation, site grading, soil, and local drainage requirements. Confirm the final layout before digging.
How deep should a French drain be?
French drain depth depends on the drainage problem, outlet elevation, pipe cover, trench detail, soil, and local conditions. Many yard-drainage French drains are planned around 18-36 inches deep, but shallow surface-water drains and deeper foundation or perimeter drains can differ. The pipe needs enough cover and a usable outlet slope. Follow the site plan, local requirements, and qualified guidance when drainage affects foundations or structures.
Is this the same as a gravel calculator?
No. A plain Gravel Calculator estimates rock volume for a bed or area. This French drain page also accounts for pipe displacement, filter fabric, slope drop, and pipe length.
Can this estimate French drain cost?
It can estimate rough DIY material cost when you enter gravel, pipe, fabric, and fixed cost inputs. It does not estimate excavation labor, site repair, permits, engineering, or contractor pricing.
Can this design drainage capacity?
No. This is a material planning calculator. Drainage capacity, stormwater rules, foundation drainage, basement waterproofing, and discharge approval depend on site conditions and should be confirmed with local guidance or a qualified professional.
Can I use this calculator with metric measurements?
Yes. Use the Imperial/Metric toggle. Inputs, results, reference values, and examples follow the active unit system where applicable.