French Drain Calculator

About the French Drain Calculator

A French drain is a gravel-filled trench with a perforated pipe that redirects surface water and groundwater away from foundations, yards, and other problem areas. It's one of the most effective solutions for wet basements, soggy lawns, and standing water issues.

This calculator estimates the gravel, perforated pipe, landscape fabric, and other materials needed for a French drain based on the trench dimensions. A typical residential French drain uses a 12–18 inch wide trench, 18–24 inches deep, with 4-inch perforated pipe surrounded by washed gravel.

Whether you're installing a foundation drain, intercepting hillside runoff, or solving a chronically wet yard, This calculator gives you accurate material quantities for ordering and budgeting.

When This Page Helps

French drains involve significant gravel volumes. A small miscalculation leads to expensive extra deliveries or an incomplete project. This calculator converts trench dimensions to exact material quantities, including the pipe displacement volume.

How to Use the Inputs

1. Enter the trench length in feet.
2. Set the trench width (12–18 inches typical).
3. Set the trench depth (18–24 inches typical).
4. Enter the pipe diameter (4 inches standard).
5. Review the gravel, pipe, and fabric estimates.
6. Calculate cost with your local material prices.

Example Calculation

Tips & Best Practices

• Use washed 3/4-inch or 1-inch gravel — never crusite (crusher run) as it packs and clogs.
• Wrap the entire trench in landscape fabric to prevent soil infiltration into gravel.
• Slope the pipe at least 1% (1/8" per foot) toward the outlet.
• Use rigid Schedule 40 perforated pipe for longevity (vs. flexible corrugated).
• Install cleanout access points every 50–100 feet for future maintenance.
• Discharge to daylight, a dry well, or a storm drain — never to a septic system.

French Drain vs. Other Drainage Solutions

French drains handle both surface and subsurface water. Surface drains (catch basins) only collect ponding surface water. Dry wells infiltrate water underground. Swales redirect surface flow. French drains are the most versatile — they collect water along the entire trench length.

Installation Steps

Dig a trench sloped 1–2% toward the outlet. Line with non-woven geotextile fabric. Add 2–3 inches of gravel along the bottom. Place perforated pipe (holes facing down or at 4/8 o'clock). Fill with gravel to within 4–6 inches of grade. Fold fabric over the gravel. Backfill with topsoil and sod.

Common French Drain Mistakes

Using the wrong gravel (crusher run clogs). Forgetting landscape fabric. Insufficient slope (water stands in pipe). Discharging into the septic system. Using corrugated pipe that crushes under backfill. Not installing cleanout access.

Maintenance Tips

Flush the system annually with a garden hose through cleanout access. Check the outlet for blockage after storms. Remove sediment from any catch basins. If flow decreases, a plumber's snake can clear the pipe. Well-maintained systems last 20+ years.

Frequently Asked Questions

• How deep should a French drain be?

  For surface water: 12‒18 inches deep. For foundation drainage: at least as deep as the foundation footing, typically 24‒36 inches. For footer drains: install at the base of the foundation wall. Deeper is better for intercepting groundwater.

• What size gravel for a French drain?

  Use washed 3/4-inch or 1-inch crushed stone. Pea gravel works but migrates more easily. Never use crusher run (road base) — the fines clog the pipe and reduce flow. River rock is acceptable for the top decorative layer.

• How much does a French drain cost?

  DIY: $5–$12 per linear foot (materials only). Professional installation: $15–$40 per linear foot. Interior basement French drains: $40–$80 per linear foot. A typical 50-ft French drain costs $250–$600 DIY or $750–$2,000 installed.

• Do French drains need a pipe?

  Yes, a perforated pipe is essential for collecting and transporting water. Without a pipe, gravel alone moves some water but far less efficiently. The pipe concentrates flow and moves it to the outlet much faster.

• How long do French drains last?

  Properly installed with fabric, washed gravel, and quality pipe: 20‒40 years. Without fabric: 5‒10 years before clogging. Rigid pipe lasts longer than corrugated. Annual flushing with a garden hose extends life.

• Where does the French drain discharge?

  Options: daylight (pipe exits on a lower slope), dry well (underground gravel pit for infiltration), catch basin connected to a storm drain, or rain garden. Never discharge into a septic system, sanitary sewer, or onto a neighbor's property.

• Do I need landscape fabric?

  Yes — non-woven geotextile fabric is critical. It wraps the gravel to prevent silt migration that clogs the system. Line the trench with fabric, fill with gravel and pipe, then fold the fabric over the top before backfilling.