French Drain Calculator

How It Works

A French drain is a gravel-filled trench with a perforated pipe that redirects groundwater away from problem areas. The trench should be at least 12 inches deep and 12 inches wide, sloping at a minimum 1% grade (1 inch of drop per 8 feet). Line the trench with landscape fabric to prevent soil from clogging the gravel. Use clean, washed 3/4-inch to 1-inch drainage gravel — never use sand or fine material that will clog the pipe. The perforated pipe goes holes-down in the bottom third of the trench. Connect the drain to a daylight outlet, dry well, or pop-up emitter away from the foundation.