How a french drain works — and what the math covers
A french drain is a gravel-filled trench that intercepts groundwater and surface water, lets it fall into a perforated pipe at the bottom, and carries it by gravity to somewhere harmless — daylight on a slope, a storm drain, or a dry well. The calculator covers the quantities: trench volume, the pipe’s displacement (a real 5–8% of the gravel on common sizes — competitors usually ignore it), the slope as an actual drop you can measure with a string line, and the dry well if one is needed. What it deliberately does not do is design the system; routing, depth relative to a foundation footing, and soil behaviour are site questions.
Gravel: how much and what kind
Use washed, angular drainage gravel around 3/4″–1.5″ (often sold as #57 stone or similar open-graded rock). “Washed” matters: fines in unwashed gravel migrate and clog the system. Density runs about 1.4 tons per cubic yard loose — editable above if your supplier quotes differently. Pea gravel works hydraulically but rounds don’t lock, making the trench surface soft underfoot; crushed is the standard choice.
Line the trench with non-woven landscape fabric before the gravel, with enough width to wrap over the top like a burrito — the fabric keeps soil out of the gravel, which is what actually determines the drain’s lifespan. As a quick rule, fabric width ≈ trench width + 2 × depth + about a foot of overlap; the result panel computes the length and suggested roll width for your trench.
Slope: the 1% rule
Water in a perforated pipe needs a continuous downhill path. The standard minimum is a 1% grade — about 1/8 inch of fall per foot of run; 2% (1/4″ per foot) is better where you can get it. Over a 40 ft trench, 1% means the outlet end is 5 inches lower than the inlet end. Two practical notes: the slope that matters is the pipe’s, set by the trench bottom — dig the bottom to grade rather than relying on gravel depth to fake it; and never let the line belly upward mid-run, since a single high spot turns the uphill half into a silt trap. The slope module converts both directions (percent ↔ inches per foot) and gives the total drop for your length.
Dry well sizing
When there is no downhill outlet, the drain ends in a dry well: a gravel-filled pit (often with a perforated tank inside) that holds a storm’s surge and lets it soak away. Sizing logic: gravel only stores water in its voids — about 40% of the pit volume for washed stone — so:
dry well volume (ft³) = gallons to hold ÷ 7.48 ÷ void ratio (≈ 0.40)
To hold 200 gallons: 200 ÷ 7.48 ÷ 0.40 ≈ 67 ft³ — roughly a 4 × 4 × 4 ft pit. A buried perforated tank improves this dramatically (its interior is 100% void), which is why commercial dry wells are tanks surrounded by gravel rather than gravel alone. The module above turns your gallons into pit volume and an example cube dimension. Soil percolation decides whether a dry well works at all — heavy clay may never empty between storms; a simple perc test (dig, fill, time the drop) is worth an hour before committing.
Gravel-only trenches
For light surface-water jobs — a soggy lawn strip, edge-of-patio interception — a gravel trench without pipe can be enough. Switch to Gravel-only mode above and the pipe subtraction disappears. The trade-off: without a pipe, water moves through gravel far more slowly (the voids are the only channel), so pipeless trenches suit short runs and modest flows. Anything carrying a downspout or protecting a structure wants the pipe.
Depth and width conventions
Typical yard drains run 12–24″ deep and 9–12″ wide — deep enough to intercept the water, wide enough to work a shovel and bed the pipe in gravel on all sides. Footing drains beside foundations go to footing depth, which is exactly the territory the safety note above hands to professionals. Whatever the depth: pipe holes generally face down in modern practice (water enters from below as the trench fills), a few inches of gravel go under the pipe, and the fabric wrap closes over the top before backfill or decorative stone.