What is a French drain?

A French drain is a gravel-filled trench containing a perforated pipe that redirects surface and groundwater away from an area. Gravel surrounds the pipe and acts as a filter, allowing water to enter while keeping soil out.

How wide and deep should a French drain be?

For yard drainage, a trench 12 inches wide and 18–24 inches deep is typical. For foundation drains, 18–24 inches wide and 24–36 inches deep is common. Deeper, wider trenches move more water but require more materials.

What size pipe should I use?

A 4-inch perforated pipe is the standard for most residential applications. For high-volume drainage or long runs, a 6-inch pipe reduces flow restriction. The calculator deducts the pipe volume from the gravel needed.

What type of gravel is best for a French drain?

Clean crushed stone (3/4 inch to 1.5 inch) or pea gravel are the most common choices. Avoid gravel with fine particles or sand, which can clog the drain over time. Washed gravel is preferred.

How much gravel do I need for a French drain?

Gravel volume equals trench length × width × depth, minus the pipe volume. Add 5–10% for waste and compaction. This calculator handles all those steps automatically.

Do I need filter fabric?

Yes, wrapping the gravel in landscape fabric (geotextile) prevents soil migration into the drain, extending its lifespan significantly. This calculator focuses on gravel and pipe quantities; order fabric to match your trench perimeter and length.

French Drain Calculator

Estimate the volume of gravel and length of perforated pipe needed for a French drain system.

Trench Dimensions

Trench Length (ft)

Trench Width (in)

Trench Depth (in)

Pipe Diameter

Waste Factor (%)

Summary

A free online French drain calculator that estimates the cubic yards of drainage gravel and the linear feet of perforated pipe required for your trench. Enter the trench length, width, and depth to get instant material estimates with an optional waste factor. Supports both Imperial and Metric units so you can plan and order materials with confidence.

How it works

Enter the total length of your French drain trench.
Enter the trench width and depth (typical: 12–24 in wide, 18–36 in deep).
Choose a pipe diameter to calculate the pipe void volume deducted from gravel.
Add a waste factor (5–10% recommended) to account for settling and spillage.
Click Calculate to see gravel volume and pipe length results.
Use the results to order materials from your local supplier.

Use cases

Planning a yard drainage French drain to redirect surface water.
Installing a foundation perimeter drain to protect a basement.
Designing a curtain drain on a hillside to intercept groundwater.
Estimating materials for a driveway edge drainage channel.
Budgeting gravel and pipe for a retaining wall drainage system.
Calculating materials for an agricultural field drainage project.

Frequently Asked Questions

Last updated: 2026-05-25 · Reviewed by Nham Vu