Gravel for a French Drain: #57 Stone, Trench Depth, Geotextile

A surface / yard French drain that clogs in three years is almost always one of two mistakes — the wrong stone, or no geotextile wrap. The math is small. The detail decisions are everything. This guide covers the DIY surface scope; true footing-depth foundation drains are specialty waterproofing work and live outside this guide.

Run any trench dimensions through the gravel calculator — pick the drainage preset and the engine defaults to a 12-inch loose depth with #57 clean stone (the surface / yard default). Output is cubic yards plus tons plus a flag when the order lands under the 5-yard supplier minimum (almost always, on residential surface drains).

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Hi, I'm the SiteworkMath assistant. I answer questions about the calculators and guides on this site — concrete yardage, topsoil, mulch, tile, and decking material math. I'm not a structural engineer or a licensed contractor; I'm a calculator built around the math that James Wu — Chicago-area flipper — uses on his own jobs. For structural decisions (joist sizing, beam spans, footing depth in your soil + climate, anything that needs a permit) talk to a licensed structural engineer or your local building department.

How pros actually do this

A gut rehab job in 2018 is the one that taught me how unforgiving the surface-drainage details are. Single-story house, mid-century build, the basement was taking water along the perimeter after every heavy rain. The previous owner had installed an interior perimeter drain in the 1990s — concrete-cut into the slab, dumping into a sump pit — to manage below-grade infiltration. What the house didn't have was any surface-water management at the perimeter: gutter discharge was dumping directly against the foundation, and the topsoil over the first 12-18 inches stayed saturated for a day after every storm.

The fix wasn't a below-grade footing drain (that scope belongs to a waterproofing contractor with shoring expertise and excavation to footing depth — 4-6 feet down on this house). It was a 90-foot surface / yard French drain at the foundation perimeter, intercepting gutter discharge and topsoil saturation before either could load the foundation wall. 12-inch deep × 12-inch wide trench: 90 × 1 / 27 = 3.33 cubic yards of stone exact, 3.75 practical at 7% cushion, 5.06 tons at #57 density. Right at the 5-yard supplier minimum if I'd called it 5 yards even — the dispatcher rounded up. One truckload, one spread day, the perforated pipe went in over a 2-inch sand bedding layer at the trench bottom, geotextile fabric wrapped the entire stone column with a top overlap before backfilling. Outlet daylighted 15 feet from the house on the downhill side.

The basement has been dry through every storm since — because the interior drain handles below-grade water and the new exterior surface drain handles surface saturation. My opinion on the two scopes: don't conflate them. A surface drain at 12 inches catches surface and shallow-saturation water and is well within DIY scope. A footing-depth foundation drain requires excavation to the footing (3-8 feet typical), waterproofing membrane against the wall, and a drain tile at the footing line — that's waterproofing-contractor territory, not a Saturday-with-a-rented-mini-excavator job.

The trap I've watched contractors fall into on surface drains is substituting crusher run because the supplier had it on hand and the operator didn't know to push back. The fines pack into the void space. The drain works fine for the first heavy rain after backfill — and then degrades steadily, and by year three the homeowner is getting water in the basement again with no obvious cause. Don't let a supplier or a contractor talk you into crusher run for drainage. #57 or equivalent angular clean stone, every time.

Which drain are you actually building?

Before the dimensions matter, get the scope right. “French drain” covers three jobs people frequently confuse:

Soggy yard, downspout dumping at the foundation, surface puddling. This is the surface / yard drain at 12 inches deep × 12 inches wide. DIY-friendly. The rest of this guide is for you.
Wet basement, footing-line water, hydrostatic pressure against the foundation wall. This is a footing-depth foundation drain (3-8 feet below grade) plus waterproofing membrane and drain tile. Waterproofing contractor scope — not a Saturday job. Get 2-3 bids; budgets typically $8-25K. This guide's 12-inch math does not apply.
No place to daylight the outlet.A French drain with nowhere to discharge isn't a drain. On a flat lot you need a sump pit + pump, or a dry well sized to the catchment area, or a discharge-to-curb permit from the municipality. Solve this BEFORE you dig the trench.

If you're in case 1 with a clear daylight outlet, the math below applies. If you're in case 2 or 3, the math doesn't fix the underlying problem — work with a waterproofing contractor or drainage designer first.

The stone choice — why #57

#57 is a No. 57 graded angular crushed stone — ¾-inch nominal with the gradation cleaned of fines. The angular shape locks loosely (enough to stay put in a vertical column) while leaving roughly 35-40% void space between pieces for water to move. Pea gravel is rounded and shifts under load, plus the void space is small enough that high-flow drainage stalls. Crusher run is angular but graded to include fines specifically to pack — the opposite of what a drain needs.

Acceptable substitutes when #57 isn't available:

#4 angular clean — 1½-inch pieces. Larger void space, slightly heavier, fine for deep drainage trenches.
#67 angular clean — ¾ to ½-inch range. Slightly tighter grading than #57, same intent.
Washed pea gravel for a shallow yard drainonly — and only when the flow rate is low enough that void-space limitation doesn't bite. Don't use pea on a foundation drain.
Crushed limestone clean — equivalent to #57 in void space and angularity, regional naming difference.

The trench dimensions

Default residential surface / yard French drain: 12 inches deep × 12 inches wide. The pipe sits in the middle of the stone column, on a 2-3 inch bed of stone underneath, with another 6+ inches of stone above and on each side. Larger pipe (6-inch) needs a wider trench (16-24 inches) to maintain the surround. Shallower yard drains (8-12 inches deep, 6-8 inches wide) work when you're only catching puddle water or downspout discharge over a short run.

A trench cross-section of 12 × 12 inches is 1 square foot, which makes the cubic-yard math friendly: every linear foot of trench is 1 cubic foot of stone, so 27 linear feet equals 1 cubic yard exact. For a 60-foot run that's 60 / 27 = 2.22 yd³; for a 90-foot run that's 3.33 yd³. Add the 7% cushion and round up to the next quarter yard.

Footing-depth foundation drains are a different job. A true exterior foundation perimeter drain has to run at or near the footing — typically 4-8 feet below grade depending on basement depth — to intercept hydrostatic pressure and groundwater before they reach the foundation wall. That scope requires excavation shoring, waterproofing membrane application, drain tile sized to the footing, backfill in compacted lifts, and sometimes a footing-side inspection. It's waterproofing-contractor scope, frequently in the $8-25K range residentially. This guide's 12-inch trench math does not apply to that scope; volume scales linearly with depth, so a 30-foot trench at 4 feet × 1 foot is 4.44 yd³ exact instead of 1.11 yd³.

The geotextile wrap

The fabric is what keeps soil fines from migrating into the gravel column. Without it, the stone slowly silts up from the surrounding earth as groundwater moves through. With it, the fabric catches the fines on the OUTSIDE of the stone column and the drainage capacity stays intact for the life of the installation.

Specification per NRCS Construction Spec 495¹:

Non-woven landscape fabric rated for drainage, minimum 4 oz per square yard.
Trench wrap. Fabric lines the trench bottom and both sides BEFORE stone goes in. Overlap fabric joins by at least 12 inches.
Top wrap. After the stone column is in and the pipe is buried, fold the fabric over the top of the column with a 6-inch overlap before backfilling. This is the part installers most often skip; without it the fines come in from above as backfill settles.
Pin the edges.Staples or landscape pins every 18-24 inches along the top edge so the fabric doesn't slip during backfill.
Don't use plastic sheet. Sheet plastic traps water against the stone instead of letting fines wash through and water move freely. The fabric is permeable on purpose.

The pipe

4-inch corrugated perforated polyethylene pipe is the residential default — that's the black coiled stuff at the big-box. The perforations face DOWN so water enters from the bottom of the holes as the trench fills with collected groundwater. Pipe goes in the MIDDLE of the stone column, on a 2-inch bedding of stone, with 6+ inches of stone above. PVC perforated pipe is stiffer but more expensive and harder to route around obstacles; corrugated black is what every drainage contractor in the Midwest uses unless the spec says otherwise.

Pipe outlet must daylight (drain to grade) or connect to a sump system. A French drain that dead-ends underground doesn't drain — it just relocates the water against a different wall or back toward the foundation. On the Chicago-suburb job above the line daylighted 15 feet from the foundation on the downhill side; on a flat lot you connect to a sump pit and pump out.

Run the math

The calculator below opens on the drainage preset (#57 clean stone, 12-inch loose depth, 30 ft × 1 ft trench — the worked example dimensions). Adjust length, width, and depth for your own trench; the engine outputs cubic yards, tons, and the supplier-minimum warning in one read.

French drain trenchMode · 12″ angular #57 stone in trench

Gravel type

Exact · 1.11 yd³

Tons · 1.69

Type · #57 / ¾″ clean

Material order cushion

The math1.11 yd³— 30 sq ft × (12″/12) / 27 = 1.11 yd³

What I’d actually order1.25 yd³or 1.69 tons at the dispatcher

Why the cushionGravel loses a fraction in transit (settling on the truck, spillage at the chute), spread inefficiency at corners and edges, and weight tolerance at the supplier (a 5-yard ticket may show up at 4.85). The cushion absorbs all three so the spread depth lands where you measured.

When NOT to over-orderBelow 5 yd³ most aggregate suppliers either won't dispatch a truck or charge a small-load surcharge ($75-150 typical). Don't talk yourself into ordering 6 yards because the math says 1 — bagged from a big-box at 0.5 cu ft per bag is the right call when the project is small.

Where this number breaks down

Stone substitution at the yard.Suppliers sometimes ship what they have on hand if the dispatch ticket isn't specific. Write “#57 clean angular, no fines” on the ticket and confirm by sight when the truck arrives. If it's graded with visible fines, send it back.
Trench too shallow.A 6-inch trench on a foundation drain doesn't intercept water that's migrating along the footing. 12 inches is the minimum for residential perimeter; deeper if the water table is high or the basement floor is more than 7 feet below grade.
Skipping the bedding under the pipe. A perforated pipe sitting directly on trench dirt collects mud inside the pipe within months. Put 2-3 inches of #57 stone UNDER the pipe before laying it.
Outlet that doesn't drain. A French drain with no daylight outlet and no sump pump is just a stone-filled trench. The water still collects; it just collects in your drain instead of in the soil. Plan the discharge before the inlet.
Mixing types in the same trench. Some installers will fill the bottom with crusher run for stability and top with #57. The fines from the bottom migrate up; the drain clogs from below. Single type, full column, every time.

Frequently asked

What size gravel is best for a French drain?

#57 angular clean stone — ¾-inch crushed rock with no fines. The angular shape gives void space for water to move through; the clean grading (no sand or fines) keeps the void space open over time. Don't use rounded pea gravel (insufficient void space for high-flow drainage) and don't use crusher run or any base aggregate (the fines pack into the flow path and the drain stops draining within a year or two). Some installers will substitute #4 or #67 in regions where #57 is hard to source; both are angular and clean and acceptable.

How deep and wide should a French drain trench be?

It depends on what the drain is doing. A yard / surface drain catching downspout discharge or surface runoff defaults to 12 inches deep × 12 inches wide — that's the common DIY scope this guide covers. A true below-grade foundation perimeter drain has to run at or near footing depth (3-8 feet below grade on a typical basement) to intercept water before it reaches the foundation wall, and that's specialty waterproofing scope, not a DIY trench. Wider (16-24″) where the trench accommodates a larger 6-inch perforated pipe instead of a 4-inch. The math for the surface case: 12 × 12 inches = 1 square foot of trench cross-section, so each linear foot of trench is 1 cubic foot. Total volume = linear feet × cross-section ft² × (1/27) for cubic yards.

Do I need geotextile fabric for a French drain?

Yes — geotextile wraps the trench AND the gravel column so soil fines from the surrounding earth don't migrate into the stone over time. Skip it and within 3-5 years the fines fill the void space, the drain stops draining, and you're digging it out and starting over. Use a non-woven landscape fabric rated for drainage applications (minimum 4 oz per square yard), overlap joins by at least 12 inches per NRCS Construction Specification 495, and wrap the fabric over the top of the stone column with a 6-inch overlap before backfilling.

What perforated pipe do I use in a French drain?

4-inch corrugated perforated polyethylene pipe (the black coiled stuff at every big-box) is the residential default. The holes face DOWN, not up — water enters from the bottom of the perforations as the trench fills, and the pipe is meant to gather the water that's already migrating through the stone column above. A 6-inch pipe is the upgrade for high-flow situations (foundation drainage on a wet site, surface drainage from a large catchment area). PVC perforated pipe is also acceptable and stiffer but more expensive. Pipe goes in the middle of the stone column, not at the bottom of the trench.

How much #57 stone do I need for a 30-foot French drain?

For a 30-foot yard / surface drain at 12 inches deep × 12 inches wide: 30 cubic feet of stone, divided by 27 = 1.11 cubic yards exact. Add the 7% cushion and round up to the next quarter-yard: 1.25 yards practical, about 1.69 tons at #57's loose density (100 lb/ft³). Below the typical 5-yard supplier minimum, so the practical sourcing is either a small landscape yard with a 2-yard minimum or bagged from a big-box at roughly 54 bags per yard (about 67 bags for this job). Deeper foundation-depth drains scale linearly — a 30-foot footing-depth trench at 4 feet × 1 foot is 4.44 cubic yards before cushion, well into bulk-delivery range. The calculator above handles any trench dimension; pick the drainage preset.

What I'd do next

Size your French drain order
Pick drainage preset, type trench length × width, get cubic yards + tons + supplier-minimum warning.
The general volume math
If your drainage scope is bigger than a perimeter trench (yard drain, dry well, retaining wall back-fill).
Foundation work in a bigger rehab
French drain is one line on a bigger basement-waterproofing budget. The flagship surfaces every line.

1. NRCS Construction Specification 495 — Geotextile — non-woven separation fabric weight (4 oz/yd²) and overlap (12-inch) specifications for drainage applications. ↩

By James Wu. Stone choice (#57 angular clean) and per-type density (100 lb/ft³ loose) trace to supplier spec sheets cross-checked against InchCalculator (1.4-1.7 tons/yd³ industry range). Geotextile specifications from NRCS Construction Specification 495 — Geotextile. Engine logic in lib/sitework/gravel.ts. Not geotechnical or basement-waterproofing advice — for high water table sites, structural foundation issues, or any drainage scope beyond standard residential perimeter, consult a licensed waterproofing contractor or a geotechnical engineer. Full methodology.

Gravel for a French drain — which stone, what depth, what wrap.