French Drain Calculator
Water pooling in your yard can turn a beautiful landscape into a muddy mess, damage foundations, and create a breeding ground for mosquitoes. A properly designed French drain is one of the most effective solutions for redirecting surface water and groundwater away from problem areas. But designing the right system requires careful planning, you need to know how much pipe, gravel, and fabric to buy, and how much the project will cost.
This French drain calculator takes the guesswork out of drainage system design. Enter the drain dimensions, pipe size, gravel depth, and material costs, the tool computes pipe length, gravel volume, fabric area, and total project cost. Whether you’re a homeowner tackling a wet yard, a landscaper installing drainage systems, or a contractor bidding on a project, this calculator delivers instant, accurate estimates. All calculations run locally in your browser, keeping your data private.
How to Use the French Drain Calculator
Enter the drain length, the distance from the start to the end of the drain system.
Specify the drain width and depth — the dimensions of the trench you’ll dig.
Enter the pipe diameter — 4 inches is standard for residential French drains.
Specify the gravel depth below the pipe — typically 4–6 inches of gravel beneath the pipe.
Set the slope percentage — 1% is typical (1 foot of drop per 100 feet of length).
Enter material costs for pipe, gravel, and fabric, plus a waste factor and labor cost.
Review the total cost and material quantities or switch to the detailed breakdown tab for a full component list.
How the French Drain Calculator Formula Works
The calculator determines the materials needed for a French drain based on the trench dimensions, pipe size, and gravel depth. The calculations follow standard drainage construction practices.
Formula: Pipe Length = Drain Length (plus waste)
The pipe length is essentially the drain length, with a waste factor applied for cuts and connections. The slope percentage doesn’t significantly affect the pipe length.
Formula: Gravel Volume = (Trench Cross-Section Area − Pipe Cross-Section Area) × Drain Length
The trench cross-section is width × depth. The pipe cross-section is π × (diameter/2)². Subtracting the pipe area gives the gravel area. This is multiplied by the drain length to get the gravel volume. Additional gravel below the pipe is also included.
Formula: Fabric Area = (2 × Width + 2 × Depth) × Drain Length
The fabric (geotextile) wraps around the trench to separate the gravel from the soil. The perimeter of the trench multiplied by the length gives the total fabric area.
Formula: Total Cost = Pipe Cost + Gravel Cost + Fabric Cost + Labor Cost
Each material cost is calculated by multiplying the quantity by the unit price, with a waste factor applied to all materials. The calculator converts gravel volume from cubic feet to cubic yards (1 yd³ = 27 ft³).
Worked Example: Residential French Drain
Consider a residential French drain project: 50 feet long, 12 inches wide, 18 inches deep, with 4-inch perforated pipe, 6 inches of gravel below the pipe, and a 1% slope. Let’s calculate the materials and cost.
Step 1: Convert units to feet
Step 2: Calculate pipe length
Pipe length = 50 ft
Step 3: Calculate trench and pipe areas
Trench area = 1.0 × 1.5 = 1.5 ft²
Pipe radius = 0.333/2 = 0.167 ft
Pipe area = π × 0.167² = 0.0873 ft²
Gravel area = 1.5 − 0.0873 = 1.4127 ft²
Step 4: Calculate gravel volume
Gravel volume = 1.4127 × 50 = 70.64 ft³
Add gravel below pipe: 1.0 × 0.5 × 50 = 25 ft³
Total gravel = 95.64 ft³ = 3.54 yd³
Step 5: Calculate fabric area
Step 6: Apply waste factor (10%) and costs
Pipe with waste: 50 × 1.10 = 55 ft
Gravel with waste: 3.54 × 1.10 = 3.89 yd³
Fabric with waste: 250 × 1.10 = 275 ft²
Pipe cost: 55 × $2.50 = $137.50
Gravel cost: 3.89 × $45.00 = $175.05
Fabric cost: 275 × $0.50 = $137.50
Total cost: $137.50 + $175.05 + $137.50 = $450.05
Interpretation: A 50-foot French drain requires about 55 feet of pipe, 3.9 cubic yards of gravel, and 275 square feet of fabric, costing approximately $450 in materials. This estimate helps you plan your project and budget accordingly.
