French Drain Calculator

A french drain is an underground trench filled with gravel and a perforated pipe that redirects excess groundwater away from a property — the French Drain Calculator estimates all the materials you need before you dig. Enter your trench length, width, and depth, then select your pipe size and standard pipe length to get the gravel volume (in cubic yards and tons), pipe pieces needed, and filter fabric area. Add your local gravel, pipe, and fabric costs to see a total material cost estimate.

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Pipe Pieces Needed

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Filter Fabric Area

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Frequently Asked Questions

What is a French drain and how does it work?

A French drain is an underground drainage system consisting of a gravel-filled trench with a perforated pipe at the bottom. Water flows into the gravel, enters the pipe through perforations, and is directed away from the area, preventing water accumulation and foundation damage.

How do I calculate the gravel needed for a French drain?

Multiply the trench length × width × depth to get the total volume, then subtract the volume occupied by the pipe. The remaining space needs to be filled with gravel. Our calculator does this automatically and converts to cubic yards for easy ordering.

What size pipe should I use for my French drain?

Most residential French drains use 4-inch perforated pipes, which handle adequate water flow for most applications. Use 3-inch pipes for lighter drainage needs or 6-inch pipes for heavy-duty commercial applications or areas with significant water flow.

How much gravel do I need per foot of French drain?

For a typical 12-inch wide by 18-inch deep trench, you need approximately 0.67 cubic feet of gravel per linear foot. This equals about 1.5 cubic yards of gravel per 100 feet of French drain.

Do I need filter fabric for my French drain?

Yes, filter fabric (geotextile) is essential to prevent soil from clogging the gravel and pipe. Wrap the fabric around the gravel-filled trench, covering the bottom and sides before backfilling with soil.

What type of gravel is best for French drains?

Use clean, angular gravel or crushed stone ranging from 1/2 to 3/4 inch in size. Avoid rounded river rock or gravel with fine particles, as these don't provide adequate drainage and can clog over time.

How do I know if my French drain is working properly?

A properly functioning French drain should eliminate standing water in the problem area within hours of rainfall. Check that water flows freely from the outlet end and that there's no backup or overflow in the trench area.

Why is it called a French drain?

The French drain is named after Henry Flagg French, a 19th-century American lawyer and farmer who popularized this drainage technique in his 1859 book 'Farm Drainage.' Despite the name, the concept of subsurface drainage has been used for centuries.