How Do I Calculate How Much Drainage Rock I Need?
Multiply trench length × width × depth (in feet), divide by 27 for cubic yards, then multiply by density for tons. Washed #57 stone weighs about 1.35 tons per cubic yard. A 50-foot French drain trench at 12×12 inches needs roughly 1.85 cubic yards or 2.5 tons.
Drainage rock is sold by the cubic yard or the ton. For French drains and footing drains, calculate the trench volume in cubic yards, then multiply by the density of your chosen aggregate (typically 1.35–1.50 tons per cubic yard for washed drainage stone).
Formula:Length (ft) × Width (ft) × Depth (in) ÷ 12 ÷ 27 = Cubic Yards. Multiply by density (tons per cubic yard) to get tons.
How Deep Should a French Drain Be?
A residential French drain is typically 12 inches wide and 18–24 inches deep. The perforated pipe sits at the bottom with 6+ inches of drainage rock all around. Always slope the trench at least 1% (1 inch per 8 feet) toward the outlet.
A residential French drain is typically 12 inches wide and 18–24 inches deep. The perforated pipe sits at the bottom with 6+ inches of drainage rock all the way around, and the trench is capped with filter fabric and backfilled to grade. For a 50-foot run at 12 inches × 18 inches of stone, expect about 2.75 cubic yards (3.7 tons of #57 stone).
Always slope the trench at least 1% (1 inch of fall per 8 feet of run) toward the drain outlet. A French drain with no slope is just a buried sponge — it holds water instead of moving it.
What Is the Difference Between #57 and #67 Stone?
#57 stone has large open voids for maximum water flow — use it for French drains, footing drains, and dry wells. #67 stone packs tighter with more fines, making it better for asphalt base and drain fields where some compaction is needed. Both weigh about 1.35 tons per cubic yard.
#57 stone is the workhorse of drainage aggregate. Washed, angular, and sized 3/4 inch to #4 sieve, it has large voids that pass water freely. Use it for French drains, footing drains, driveway base, and dry wells.
#67 stone is the same size range but contains more fines in the blend, so it packs tighter. Use it under asphalt base and in drain fields where some compaction is desired. Both grades weigh approximately 1.35 tons per cubic yard loose.
How Deep Should Drainage Rock Be Around a Foundation?
Surround the 4-inch perforated pipe with at least 6 inches of drainage stone on all sides — 14–16 inches total rock depth. For a 140-linear-foot foundation perimeter at 12×16 inches of stone, you need about 6.9 cubic yards (9.3 tons) of #57 stone.
A perimeter footing drain is a 4-inch perforated pipe laid at the exterior base of the foundation footing, surrounded by at least 6 inches of clean drainage stone on all sides. Total rock depth is typically 14–16 inches. Wrap the rock zone in non-woven filter fabric to keep silt and fines out of the voids.
For a 40-foot × 30-foot foundation perimeter (140 linear feet), at 12 inches × 16 inches of stone coverage, you need approximately 6.9 cubic yards (9.3 tons) of #57 stone.
Do I Need Filter Fabric Under Drainage Rock?
Yes — non-woven geotextile filter fabric is essential for any long-term drainage installation. Without it, fine soil migrates into the rock voids and clogs them within a few years. Overlap seams by at least 12 inches. Woven fabric is not a substitute.
Is Bulk or Bagged Drainage Rock Cheaper?
Bulk drainage stone runs $35–60 per cubic yard delivered; bagged is 3–5× more expensive. Use bags only for small patch-up jobs or when delivery access is impossible. For anything over 1 cubic yard, bulk delivery saves significant money.
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Written by Daniel McCarney — AceCalc