French Drain Gravel Calculator
A french drain is a gravel-filled trench with a perforated pipe that moves water away from where it's causing trouble. Most guides tell you the trench size but leave you to figure out the stone. This calculator does the volume math β including subtracting the pipe β and converts to tons of washed drainage gravel.
How this calculator works
Gravel volume = trench volume (length Γ width Γ depth) minus pipe volume (Ο Γ radiusΒ² Γ length), converted to cubic yards and tons at 1.4 tons per yard. Use washed, rounded 3/4"β1.5" drainage stone β crusher run and anything with fines will clog. Wrap the trench in geotextile fabric so soil doesn't silt the stone bed.
Frequently asked questions
How much gravel do I need per foot of french drain?
For the standard 12" wide Γ 18" deep trench with a 4" pipe: about 1.41 cu ft of stone per linear foot, or roughly 1 ton per 14 ft of trench. A 30 ft drain therefore takes about 2.2 tons.
What kind of gravel goes in a french drain?
Washed round stone, 3/4" to 1.5", often sold as drainage rock or #57 washed gravel. Never use crusher run, limestone screenings, or anything dusty β the fines cement together and defeat the drain.
How deep should a french drain be?
18β24" for surface water problems; footing drains around a foundation go down to the footing itself. Keep a minimum 1% slope (1" of fall per 8 ft) toward the outlet β a drain without fall is just a wet trench.