Multiply wall length × height × thickness in feet, divide by 27 for cubic yards, then multiply by 1.6 for tons of wall stone. A 20 ft long, 3 ft high wall at 24 inches thick needs about 4.4 cubic yards — roughly 7.1 tons — plus another 3.2 tons of washed stone for the 12-inch drainage column behind it.
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Retaining Wall Calculator Guide
A retaining wall is really two stone orders wearing one name. The first is the wall itself — the visible face you enter here as length × height × thickness, returned in cubic yards and tons at a 15% waste allowance. The second is invisible once the job is done: a 12-inch-wide column of washed drainage stone running up the back of the wall, and it is the order most first-time builders forget entirely.
That hidden column is not optional decoration. Retaining walls rarely fail because the stone was too weak; they fail because saturated soil behind them turned into hydraulic pressure with nowhere to go. The drainage stone and the pipe at its base are the pressure-relief valve, which is why an honest estimate prices both orders from the start.
How Much Stone Does a Retaining Wall Need?
| Wall height | Wall stone per 10 ft (24 in thick) | Drainage stone per 10 ft (12 in column) |
|---|---|---|
| 2 ft | ≈ 1.5 yd³ / 2.4 tons | ≈ 0.75 yd³ / 1.1 tons |
| 3 ft | ≈ 2.2 yd³ / 3.6 tons | ≈ 1.1 yd³ / 1.6 tons |
| 4 ft | ≈ 3.0 yd³ / 4.7 tons | ≈ 1.5 yd³ / 2.1 tons |
Wall stone figures use gabion-grade rock at 1.6 tons per cubic yard; the drainage column uses washed #57 at 1.45. To price the drainage order, run the calculator a second time with the wall height unchanged, thickness set to 12 inches, and #57 selected. Four feet is also the ceiling: any wall taller than that — or any wall with a driveway, slope, or building leaning on it from above — belongs to an engineer, not a calculator.
Batter, Base, and Weep Drainage
- Dig the base trench first. Excavate below topsoil and compact 6 inches of crusher run as a footing pad, then bury the entire first course of stone — roughly 10% of the wall height below grade anchors the wall against kick-out.
- Build with batter. Step each course back about 1 inch per foot of height so the wall leans into the slope it restrains. A plumb retaining wall is a wall waiting for its first wet spring.
- Lay the weep system early. Perforated pipe goes at the bottom of the drainage column, holes down, sloped to daylight past the wall ends before any backfill rises above it.
- Fabric between soil and stone. Filter fabric on the soil side keeps fines from silting the drainage column shut over the years — clogged stone behaves like no stone at all.
- Backfill in lifts. Alternate drainage stone against the wall and native soil behind the fabric, compacting lightly every 8 inches rather than dumping the whole depth at once.
Size the compacted footing pad separately with the crusher run calculator, and use the #57 stone calculator if you prefer to price the drainage column as its own line item. Ordering all three materials in one delivery usually beats three small-load fees — and guarantees the drainage stone actually shows up on build day.
Frequently Asked Questions
How much drainage stone goes behind a retaining wall?
Plan on a 12-inch-wide column of washed stone running from the base of the wall to within a few inches of the top. For every 10 feet of a 3-foot wall that is about 1.1 cubic yards — roughly 1.6 tons of #57 stone.
What is batter on a retaining wall?
Batter is the deliberate backward lean into the slope, typically 1 inch of setback per foot of height for dry-stacked stone. It shifts the wall’s center of gravity into the retained soil so the load pushes the stones together instead of tipping them out.
How tall can I build a retaining wall without an engineer?
Most codes cap unengineered walls at 4 feet measured from the bottom of the base. Anything taller — or any wall carrying a driveway, slope, or structure above it — needs an engineered design, and terraced walls count as one tall wall unless they are set back at least twice the height of the wall below.
Does a retaining wall need a drain pipe?
Yes. Lay a perforated pipe at the base of the drainage column, sloped to daylight at the wall ends, and separate the washed stone from native soil with filter fabric. Trapped water is what pushes retaining walls over — the stone and pipe give it somewhere to go.