Gravel for a French drain: which size, how deep and how much

A French drain is a trench of clean stone, usually around a perforated pipe, that collects water and carries it to a lower outlet. The stone has two jobs: let water in quickly, and stay larger than the slots in the pipe. The sizes below come from highway and building specifications; the quantities and prices come from the #57 weights and list prices in our ledgers.

Which gravel size to use

Texas DOT’s 2024 specification for pipe underdrains lists five filter materials, then says to “use Type G filter material around the underdrains unless otherwise shown on the plans.” Type G is Grade 4 (57) coarse aggregate: #57 stone. Maryland’s stormwater materials guide calls No. 57 “the most widely used aggregate in SWM construction”, including the stone around underdrains.

For foundation drains, the International Residential Code asks for washed gravel or crushed rock “not less than one sieve size larger than the tile joint opening or perforation” (R405.1). A typical 4 in single wall drain pipe has slots at most 0.120 in (3 mm) wide. The table compares that slot with the two sizes most often sold for drains.

#57 ends at a sieve opening more than one and a half times the slot width; #8 ends below it. The specifications care more about size and cleanliness than shape: the IRC accepts washed gravel or crushed rock, and TxDOT’s filter material may be “sand, gravel, crushed stone, or crushed shell” that meets the grading. Washed matters: TxDOT limits loss by decantation to 1% of the material retained on the No. 4 sieve.

How deep the stone goes

R405.1 sets the pipe on at least 2 in of stone and covers it with at least 6 in. ADS lists 4.6 in as the average outside diameter of its 4 in single wall pipe, so the least stone that meets the code around a 4 in pipe is 2 + 4.6 + 6 = 12.6 in.

Highway underdrains go deeper. TxDOT places filter material at least 12 in above the bottom of the pipe, which on a 2 in bed makes 14 in of stone. Around a house foundation, R405.1 also says a gravel or crushed stone drain extends at least 1 ft beyond the outside edge of the footing and 6 in above its top, and that the drain discharges by gravity or mechanical means into an approved drainage system.

How much #57 stone a French drain takes

Stone volume is the trench cross-section minus the pipe, times the length. At the two #57 selling weights in our ledger, 2,400 and 2,410 lb per cubic yard (recorded September 23, 2026), the two answers differ by less than 0.1 ton even at 100 ft.

Suppliers sell #57 by the ton or by the yard, often in half or whole units, so round up to their increment. For a trench not in the table, use the #57 stone calculator with the trench length, width and depth; it does not subtract the pipe, which takes about 0.12 ft³ of each foot of trench.

What the stone costs

These are the #57 list prices in our ledger, recorded September 23, 2026. Prices exclude delivery unless the terms say otherwise.

At $21.50 to $69 a ton, the stone for a 50 ft drain in a 12 × 18 in trench (3.1 tons) comes to $66 to $213 before delivery. On a load this small, delivery can cost more than the stone; the dump truck load guide shows how the price per ton falls with order size.

Fabric and pipe placement

TxDOT lines the trench bottom and sides with filter fabric before the pipe and stone go in, then laps the fabric over the top of the stone. It centers the perforated pipe in the trench “with the perforations below the horizontal axis.” For foundation drains, R405.1 asks for an approved filter membrane over the stone or around the pipe.

To wrap the full stone section, fabric must be at least twice the trench width plus twice the stone depth, before the overlap the fabric maker specifies: 49 in for the 12.6 in section, 60 in for 12 × 18 in and 72 in for 12 × 24 in.

How much water the stone holds

Stormwater manuals size #57 beds at 40% voids: Virginia DEQ uses a porosity of 0.4, and the Minnesota Stormwater Manual asks for a void ratio of at least 40%. At that porosity, plus the inside of a 4 in pipe, 10 ft of a 12 × 18 in drain holds about 48 gallons as it fills; the IRC minimum section holds about 34 and an 18 × 24 in trench about 93. That is storage while the trench fills, not how fast the pipe carries water away.

How these figures were worked out

• Stone per foot of trench (ft³) = width × depth ÷ 144 − π × (4.6 ÷ 24)², the pipe’s outside area.
• Tons = ft³ ÷ 27 × 2,400 or 2,410 lb/yd³ ÷ 2,000. No compaction or waste allowance.
• Water held per foot (gallons) = (stone ft³ × 0.4 + π × (4 ÷ 24)²) × 7.48.
• Fabric width = 2 × width + 2 × depth; add the manufacturer’s overlap.