French Drain Calculator

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.

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.

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.

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.

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.

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.

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.

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.