French Drain Gravel Calculator
Estimate the drainage gravel for a French drain trench — cubic yards and tons after subtracting the perforated pipe.
How it works
gravel = [ length × (width ÷ 12) × (depth ÷ 12) ] − pipe volume · pipe = π × (dia ÷ 24)² × length
A French drain is a gravel-filled trench with a perforated pipe at the bottom, so the stone volume is the trench box minus the cylinder the pipe occupies. Use clean, angular ¾ in washed drainage stone — it will not compact shut the way pea gravel or fines do. Drainage stone weighs roughly 1.4 tons per cubic yard, so both a cubic-yard and a tonnage figure are given for ordering.
Worked example
A 50 ft trench 12 in wide and 18 in of gravel deep is 75 cu ft. A 4 in perforated pipe displaces π × (2 in)² × 50 ft = 4.4 cu ft, leaving 70.6 cu ft = 2.61 cubic yards, about 3.7 tons.
Frequently asked questions
What gravel is used in a French drain?
Clean, angular washed stone, usually ¾ in to 1½ in, with no fines. Rounded pea gravel and crusher-run pack down and defeat the drain; the gaps between angular stone are what carry the water.
Do I need filter fabric?
Yes — line the trench with a non-woven geotextile and wrap it over the top of the stone. It keeps soil fines from migrating in and silting up the voids, which is the main way French drains fail over time.
Which way do the pipe holes face?
Point the perforations down. Water rises into the pipe from below through the gravel; holes-down lets the pipe collect water at the lowest point and carry it to the outlet. Slope the whole run at least 1% to daylight.
Related calculators
Estimates only — add for waste and confirm against local building codes and supplier specs.