How a french drain works
A french drain is a gravel-filled trench, usually with a perforated pipe running along the bottom, that gives water an easy path to follow instead of letting it pool or push against a foundation. Water moves through soil by taking the path of least resistance — gravel has far more open space between the stones than compacted soil does, so water entering the trench flows into the gravel and along the pipe (or along the gravel itself, if there's no pipe) toward a lower discharge point, rather than soaking sideways into the ground it's supposed to be draining.
The formula, and a worked example
The gravel volume is the trench volume minus whatever space the drain pipe takes up, with a waste allowance added on top for spillage and uneven trench walls:
Gravel (ft³) = [L × (W ÷ 12) × (D ÷ 12) − π × (pipe dia ÷ 24)² × L] × (1 + waste)
Length stays in feet while width and depth convert from inches by dividing by 12; the pipe term uses the same length-in-feet convention, with its diameter converted from inches to a feet-radius by dividing by 24. For example, a 30 ft trench, 12 in wide and 18 in deep, with a 4-in pipe and 5% waste:
30 × 1 × 1.5 = 45 ft³ trench − 2.62 ft³ pipe ≈ 42.4 ft³, then ×1.05 waste ≈ 44.5 ft³ ≈ 1.65 yd³ ≈ 2.3 tons
That 2.3 tons is what a supplier would quote for a bulk delivery. Converting to 0.5 cubic foot bags (the common bag size at home improvement stores) gives 90 bags for this example — a reminder that bagged gravel gets expensive fast on anything beyond a short trench, and bulk delivery is worth it past roughly 1 ton.
What gravel to use
Washed round gravel or washed crushed stone, sized around ¾ inch to 1½ inches, is the standard fill for a french drain. The washing is the important part — it strips out the fine sand and silt that would otherwise settle into the gaps between stones and slowly choke off the drain's capacity to move water. Avoid products sold for compaction, like crusher run, road base, or limestone screenings: those are blended with fines on purpose so they pack tight, which is the opposite of what a drain needs.
Wrapping the trench (and often the pipe) in landscape or filter fabric before backfilling with gravel is standard practice too. It keeps the surrounding soil from migrating into the gravel over time — without it, even clean washed stone will eventually silt up from the soil pressing in around it.
Typical trench dimensions
Most residential french drains use a trench around 12 inches wide and 18 to 24 inches deep — wide enough to work in with a shovel, and deep enough to sit below a shallow foundation footing or the local frost line in many areas. Narrower or shallower trenches are fine for simple yard drainage moving surface water away from a low spot; deeper trenches are usually about intercepting groundwater near a structure.
Slope matters as much as depth. A french drain needs a consistent downhill grade to a discharge point — a minimum of about 1% grade, roughly 1 inch of drop for every 8 to 10 feet of trench run. Less slope than that and water can sit in the pipe instead of moving through it, especially once the gravel starts collecting any sediment.