How Much Gravel for a French Drain?

Updated 2026-07-19

A French drain is simple physics: water follows the easiest path, so you give it one — a gravel-filled trench with a perforated pipe at the bottom that carries runoff away from the foundation, the soggy lawn, or the patio that floods every April. The part homeowners consistently underestimate is not the digging. It is how much gravel a trench swallows.

Here is the stone to buy, the trench math to run, and the build order that keeps a French drain working for decades instead of clogging in three years.

Use 3/4-inch washed stone — nothing else

The gravel in a French drain has one job: leave big, open voids so water moves fast and silt has nowhere to settle. That means 3/4-inch washed stone, sold as #57 at most quarries. Washed is the key word — no dust, no fines, no "minus" in the name. Crusher run or road base may be cheaper by the ton, but the fines that make it compact beautifully under a driveway will plug your drain's voids exactly the same way.

Skip pea gravel too. It is clean enough, but the small rounded size packs tighter than #57 and usually costs more per yard. For a drain, boring, angular, washed 3/4-inch stone is the professional default for a reason.

Trench math: how many tons?

A standard residential French drain trench is 12 inches wide and 18 inches deep — enough to hold a 4-inch perforated pipe with 2 to 3 inches of gravel below it and stone all around it. A 50-foot run at that cross section holds 75 cubic feet; subtract about 4.5 cubic feet for the pipe itself and you need roughly 70 cubic feet of gravel — about 2.6 cubic yards, or 3.5 to 4 tons at typical washed-stone weight.

For quick estimates, remember it this way: a 12-by-18-inch trench eats about 3/4 of a ton of gravel for every 10 feet of length. Doubling to a 100-foot run means 7 to 8 tons — the moment bagged stone from the big-box store stops making sense and a bulk delivery pays for itself.

Build order that keeps it draining

1. Check slope first. The pipe needs at least 1 percent fall — 1/8 inch per foot — and 2 percent (1/4 inch per foot) drains noticeably better. A 50-foot run should drop at least 6 to 12 inches end to end.
2. Line the trench with non-woven geotextile fabric, leaving enough overhang on both sides to wrap over the top later.
3. Pour a 2 to 3 inch gravel bed and set the 4-inch perforated pipe on it, holes facing down — water rises into the pipe from below.
4. Fill around and over the pipe with washed stone to within 3 to 4 inches of grade.
5. Fold the fabric over the top like a burrito so soil cannot wash down into the stone, then finish with topsoil and sod — or decorative rock where the trench stays visible.

Where does the water go?

Every French drain needs a legal exit. Daylighting the pipe on a downhill slope is best; a dry well — a fabric-lined gravel pit of 10 cubic feet or more — works where the lot is flat. Discharging onto a neighbor's property or tying into a sanitary sewer violates code in most municipalities, so check before you dig. And call 811 for utility locates a few days ahead; it is free, and it is the law.

Keep the outlet clear of mulch and leaves, keep downspouts from dumping directly onto the trench line, and a fabric-wrapped #57 drain will quietly do its job for 30 years or more.

Get the tonnage right the first time

The most common French drain failure is not clogging — it is the job stalling half-dug because the gravel ran out at foot thirty. Trench volume is unforgiving math, and rock yards do not deliver half tons on a Saturday afternoon.

Before you order, measure your trench length and run the width and depth through the gravel calculator — it converts straight to tons and cubic yards, the same units your quarry quotes in, so no 4-ton surprise shows up on the invoice.