French drain materials: gravel, pipe, and fabric by the numbers

You've dug the trench, it's a clean 50 feet long, a foot wide, a foot and a half deep — and now you're standing at the gravel yard trying to figure out how many tons to order. If you just multiply the trench's outer dimensions, you'll over-order, because a good chunk of that trench is going to be filled with a perforated pipe, not stone. Order gravel for the pipe's own volume and you've paid for rock that was never going to fit.

A French drain is one of the simpler drainage fixes a homeowner can build, but the materials math has three quiet traps: gravel that ignores the pipe taking up space, landscape fabric that gets skipped because it "doesn't do anything visible," and depth/slope numbers that get eyeballed instead of measured. None of them are complicated once you see them, but all three change what shows up on the receipt.

Gravel volume has to subtract the pipe

The trench itself is a rectangular volume: length times width times depth. But the pipe running down the middle of that trench is a cylinder of dead space — gravel can't occupy the volume the pipe physically fills. For a 4-inch pipe, that's a 2-inch-radius cylinder running the full length of the trench, and over 50 feet that adds up to several cubic feet of gravel you'd otherwise be buying and never using.

The correct volume is: trench volume minus pipe volume. For a cylinder, that's length × π × radius², with the pipe radius converted to feet. It's a small percentage of the total on a wide trench with a narrow pipe, but on a tight 12-inch-wide trench with a fat 4-inch pipe, the pipe is genuinely eating a noticeable slice of the cross-section — enough that ignoring it means over-ordering by a meaningful margin on a multi-ton delivery.

Why gravel is priced and delivered by weight, not volume

Gravel yards sell by the ton, not the cubic yard, because different stone (washed drainage rock vs. crushed limestone vs. pea gravel) packs to a different density. A typical washed drainage gravel runs somewhere around 100 lb per cubic foot loose, but that number moves with stone size and moisture, so it's worth confirming with your supplier rather than assuming. Converting your net gravel volume (in cubic feet) to tons means multiplying by the density and dividing by 2,000 lb — skip that conversion and you'll either under-order (paying return trips) or over-order (paying for gravel sitting in a driveway pile).

Landscape fabric: cheap now, expensive to skip

Fabric wrapped around the gravel — or lining the trench before backfill — keeps soil and fine silt from migrating up into the gravel voids over time. Without it, a French drain that works perfectly for the first few seasons gradually silts in from the sides and bottom, and the only fix at that point is digging the whole thing back up. Fabric costs a small amount per linear foot and is the cheapest insurance in the entire project relative to the labor cost of redoing a failed drain years later. It's easy to leave off the shopping list because it doesn't show up in a "how much gravel do I need" calculation, but it belongs in the total cost from the start.

Depth and slope conventions

A few rules of thumb keep a French drain actually draining instead of just holding water:

• Depth: most residential French drains run 18–24 inches deep — enough to get below frost heave in cold climates and below the zone where surface roots and foot traffic disturb the trench, but not so deep that you're fighting bedrock or utilities.
• Slope: the trench needs a consistent fall toward the outlet, conventionally around 1% grade (roughly 1 inch of drop per 8 feet of run). Too flat and water sits in low spots inside the pipe instead of moving; there's no such thing as "too much slope" within reason.
• Pipe orientation: perforated pipe goes with holes facing down (into the gravel) in most designs, so water enters from below and gravity feeds it along, rather than holes-up designs which let sediment settle into the perforations from above.

Worked example

A 50 ft trench, 12 inches wide, 18 inches deep, with a 4-inch perforated pipe:

• Trench volume: 50 ft × (12/12) ft × (18/12) ft = 75 cu ft
• Pipe volume: 50 ft × π × (2/12 ft)² ≈ 4.36 cu ft
• Net gravel volume: 75 − 4.36 ≈ 70.64 cu ft, or about 2.62 cu yd
• Gravel weight at 100 lb/cu ft: 70.64 × 100 ÷ 2,000 ≈ 3.53 tons

At $45 per ton, that gravel runs about $158.93. Add 50 ft of 4-inch perforated pipe at $1.20/ft ($60) and 50 ft of landscape fabric at $0.50/ft ($25), and the full materials bill lands around $243.93 — with the pipe subtraction alone accounting for roughly $6 of gravel you didn't need to buy. On a longer or narrower trench with a fatter pipe, that subtraction gets proportionally larger.

Order the right amount, once

Gravel deliveries usually come with a minimum order and a delivery fee, so guessing low and making a second call is expensive, and guessing high leaves you with a stone pile to deal with. Our French drain calculator takes your trench length, width, depth, and pipe diameter and returns the net gravel volume (already subtracting the pipe), the weight in tons, and the full materials cost including pipe and fabric — so the number you call in to the gravel yard is the number you actually need.