French Drain Calculator for Construction Drainage Planning

Calculator Inputs

The page stays single-column overall, while the form uses a responsive 3-column, 2-column, and 1-column layout.

Unit System

Drain Length (ft)

Trench Width (ft)

Trench Depth (ft)

Pipe Diameter (in)

Pipe Length (ft)

Slope (%)

Fabric Overlap (in)

Gravel Void Ratio (%)

Waste Allowance (%)

Gravel Density (lb/ft³)

Excavation Cost ($/ft³)

Gravel Cost ($/short ton)

Pipe Cost ($/ft)

Fabric Cost ($/ft²)

Example Data Table

This example matches the default imperial values loaded in the form.

Drain Length	Width	Depth	Pipe Diameter	Slope	Void Ratio	Waste	Adjusted Gravel	Water Storage	Total Cost
50 ft	1.5 ft	2 ft	4 in	1%	40%	10%	160.20 ft³	58.25 ft³	$1,617.40
30 ft	1 ft	1.5 ft	4 in	1%	35%	8%	47.48 ft³	15.39 ft³	Sample scenario for tighter residential runs

Formula Used

1) Trench Volume
Trench Volume = Length × Width × Depth

2) Pipe Fall
Pipe Fall = Pipe Length × (Slope % ÷ 100)

3) Pipe Displacement
Pipe Displacement = π × (Pipe Diameter ÷ 2)² × Pipe Length

4) Net Gravel Volume
Net Gravel Volume = Trench Volume − Pipe Displacement

5) Adjusted Gravel Volume
Adjusted Gravel Volume = Net Gravel Volume × (1 + Waste Allowance)

6) Fabric Area
Fabric Area = Pipe Length × (Width + 2 × Depth + 2 × Overlap) × (1 + Waste Allowance)

7) Water Storage Capacity
Water Storage = Net Gravel Volume × Void Ratio

8) Total Estimated Cost
Total Cost = Excavation Cost + Gravel Cost + Pipe Cost + Fabric Cost

These calculations provide planning estimates. Final drainage design should still consider soil conditions, outlet capacity, code rules, and site-specific engineering.

How to Use This Calculator

Select imperial or metric units before entering values.
Enter drain length, trench width, trench depth, and pipe diameter.
Set the pipe length and desired slope percentage.
Add fabric overlap, gravel void ratio, and waste allowance.
Enter gravel density and unit costs for excavation, gravel, pipe, and fabric.
Press Calculate French Drain to show results above the form.
Review the quantities, cost cards, and Plotly graph.
Use the CSV and PDF buttons to export the completed estimate.

Frequently Asked Questions

1) What does a French drain calculator estimate?

It estimates trench volume, gravel quantity, perforated pipe drop, filter fabric area, storage capacity, and a rough material and excavation cost for planning purposes.

2) Why is slope important in a French drain?

Slope helps move collected water toward the outlet. Too little slope can slow drainage, while uneven installation can create low spots that trap sediment and reduce performance.

3) What slope is commonly used for French drains?

Many installers use about 1% slope, or roughly 1 foot of fall per 100 feet. Actual needs depend on site conditions, outlet level, and local practice.

4) Why does the calculator subtract pipe displacement?

The pipe occupies space inside the trench. Subtracting its volume gives a more realistic gravel quantity, which improves both ordering accuracy and cost estimation.

5) What does gravel void ratio mean?

Void ratio represents the open space between stone pieces. That open space temporarily stores water, so it directly affects the drain’s available storage capacity.

6) Why add waste allowance to gravel and fabric?

Waste allowance covers overlap, trimming, settlement, handling losses, and uneven trench conditions. It helps prevent under-ordering and reduces jobsite delays.

7) Does this calculator replace engineering design?

No. It is a planning and estimating tool. Soil infiltration, hydrostatic pressure, building setbacks, code requirements, and outlet conditions still need professional review.

8) Can I use this for both residential and commercial work?

Yes, for preliminary estimating. Larger sites may require hydraulic review, staged collection systems, catch basins, cleanouts, and coordinated grading beyond simple trench calculations.