French Drain Calculator

How to estimate materials for a french drain

A french drain is one of the most reliable ways to move unwanted water away from a wet basement wall, a soggy yard, or a low spot that will not dry out. Sizing the trench correctly and ordering the right amount of gravel and pipe up front makes the whole project go faster and cheaper.

What a french drain is

The concept is simple. You dig a trench, line it with landscape fabric, place a perforated pipe at the bottom, then fill the trench with washed drainage gravel and wrap the fabric back over the top. Water enters the gravel from above and below, drops down through the holes in the pipe, and flows to a lower discharge point.

Trench dimensions

Most residential french drains are about 12 inches wide and 18 to 24 inches deep. Deeper trenches capture more subsurface water and reach a stronger flow zone. For foundation applications, the trench often follows the footing depth. Wider is not always better, since wider trenches waste gravel without collecting proportionally more water.

The gravel volume math

Trench volume is length times width times depth, with all three in the same units. Width and depth are usually in inches, so divide each by 12 to convert to feet. Multiply the three feet values together for cubic feet, then divide by 27 to get cubic yards, which is how gravel is sold. This calculator handles the conversions for you.

Pipe length matches the trench

Perforated drain pipe runs the full length of the trench, so pipe linear feet equals trench length. Buy a little extra for connections to solid pipe at the outlet and any fittings. Four inch corrugated perforated pipe is the most common residential choice.

Slope makes it work

A french drain only works if water actually flows downhill through the pipe. Plan for at least 1 percent slope, which is 1 inch of drop per 10 feet of run. Without slope, water sits in the pipe and the drain becomes little more than a buried gravel pit.

These are planning estimates. A drainage contractor will confirm final materials after seeing the site and outlet location.