French Drain Calculator: How Much Gravel and Pipe Do You Need?
A French drain is one of the most effective solutions for yard drainage problems, wet basements, and waterlogged soil. Calculating the materials correctly before you rent an excavator saves time and money. This guide explains what a French drain is, how to size it, and how to calculate gravel and pipe quantities.
What Is a French Drain?
A French drain is a gravel-filled trench containing a perforated pipe that redirects surface water and groundwater away from problem areas. Water seeps into the gravel, flows into the perforated pipe, and is carried downhill to a suitable outlet (ditch, dry well, or daylight). They’re used to relieve hydrostatic pressure against foundations, eliminate standing water in yards, and drain soggy areas.
Typical French Drain Dimensions
A standard residential French drain trench is 12-24 inches wide and 12-24 inches deep. The perforated pipe is typically 4-inch diameter for most applications (6-inch for heavy drainage). The trench is filled with washed gravel (#57 stone is commonly used). The pipe sits at the bottom of the gravel bed with holes facing down.
Calculating Gravel Volume
For a 50-foot trench that is 18 inches wide and 18 inches deep: volume = 50 ft × 1.5 ft × 1.5 ft = 112.5 cubic feet. Subtract the pipe volume (negligible for 4-inch pipe). Convert to cubic yards: 112.5 ÷ 27 = 4.2 cubic yards. Gravel compacts approximately 10-15%, so order 4.7-5 cubic yards. At $35-$55 per cubic yard delivered, expect to spend $165-$275 on gravel for this project.
Pipe Selection
4-inch corrugated perforated pipe is the standard choice for most residential applications. It comes in 10-foot and 100-foot rolls. Solid pipe is used at the outlet end and any sections you don’t want to collect water. Use perforated pipe for the collection zone and transition to solid pipe near the outlet to prevent clogging there.
Landscape Fabric
Line the trench with water-permeable landscape fabric before adding gravel. Wrap the fabric around the entire gravel fill (including the top). This prevents soil from migrating into the gravel over time and clogging the drain. Use fabric rated specifically for drainage applications — the open-weave type, not the woven landscape weed barrier.
Calculate your drainage materials. Use the French Drain Calculator on diycalculator.net to get exact gravel, pipe, and fabric quantities for your specific trench dimensions.