French Drain Gravel Calculator — How Much #57 Stone?
Pre-set for washed #57 stone (≈1.4 tons/yd³). Enter your trench as length × width × depth — the pipe displaces only a small fraction, and the calculator's 5–10% allowance covers it.
Gravel calculator
Density shown is an industry-typical value and varies by supplier — confirm before ordering.
You need
Includes +10% extra
36.0
cubic feet
1.33
cubic yards
1.87
tons (2,000 lb)
72
bags (0.5 cu ft)
≈ 0.76 m³
Estimated cost: $0.00 (at $0/ton)
Order 5–10% extra for compaction and settling — loose gravel always settles to less volume after compaction.
How much gravel does a French drain need? A 50-foot trench, 12 inches wide and 18 inches deep, takes 3.33 cubic yards of washed #57 stone — about 4.7 tons. That assumes a 4-inch perforated pipe surrounded by stone. Enter your trench's length, width and depth above; the +5% allowance covers the volume the pipe displaces.
How a French drain is layered (and what to calculate)
A French drain is a gravel-filled trench with a perforated pipe at its bottom — the stone is the drain, and the pipe just speeds the water along. Water enters the voids between stones, drops to the pipe, and flows to daylight or a dry well, so aggregate choice and trench size — not pipe brand — decide performance. The classic residential cross-section, top to bottom:
- Trench — commonly 18–24 inches deep and 9–12 inches wide, dug with ~1 inch of fall per 8 feet of run.
- Bedding — 2 inches of #57 stone in the base.
- Pipe — 4-inch perforated PVC or corrugated, holes down (or as the manufacturer specifies), sleeved in filter fabric in silty soils.
- Backfill — #57 stone to grade; top with soil and sod, or leave stone exposed as a channel.
Calculate the whole trench volume — length × width × depth — and don't subtract the pipe. A 4-inch pipe occupies about 0.35 cubic feet per linear foot (roughly 5% of a 12-inch-wide trench), which the calculator's +5% setting handles exactly.
Why washed #57 — and what not to use
Washed #57 crushed stone (3/8–1 inch, no fines) is the standard French drain aggregate because its angular voids pass water freely. The "washed" matters: fines and dust migrate, clog, and kill drainage within a few seasons.
- Use: washed #57, #67, or clean 3/4-inch stone (≈1.4 t/yd³).
- Avoid: crusher run / #411 — the fines that make it compact also make it nearly waterproof.
- Avoid: pea gravel and river rock — rounded stone packs tighter voids; second choice.
- Never: sand, fill dirt, or recycled concrete with fines.
Grades and sizing background is in our gravel size chart.
Getting the fall right
Slope the trench about 1 inch per 8 feet (roughly 0.5–1%) toward the outlet — too little fall silts up, too much moves water without filtering. Check with a line level as you dig, and before backfilling, run a hose into the trench: water should drain through the stone visibly within minutes. Outlets matter as much as slope: daylight the pipe somewhere water can leave safely (a swale, the street, or a pop-up emitter). A French drain that ends nowhere is an expensive French puddle.
Frequently asked questions
What size gravel goes in a French drain?
Washed #57 crushed stone — about 3/8–1 inch, no fines — is the standard French drain aggregate. The voids between angular stones let water flow freely; sand and crusher-run blends clog the drain.
How much gravel do I need around a 4-inch pipe?
A common rule is at least 2 inches of stone beneath the pipe and 2–3 inches above and beside it, in a trench 9–12 inches wide. Enter your trench length, width and total depth in the calculator above.
How deep should a French drain trench be?
Most residential French drains are 18–24 inches deep to intercept groundwater, sloped about 1 inch per 8 feet of run. Depth follows your water problem and frost line, so adjust the calculator’s depth to match your trench.