How much gravel do I need behind a retaining wall?

For a 12-inch-wide column behind the wall: multiply wall length × wall height (both in feet) to get the volume in cuft. Divide by 27 for cubic yards. A 30-ft, 4-ft-tall wall needs about 4.4 yd³ or 6 tons.

What kind of gravel goes behind a retaining wall?

#57 washed crushed stone. Open-graded for fast drainage, washed so dust doesn't clog the perforated drain pipe at the base, angular so it locks together and doesn't shift over time.

Can I use pea gravel behind a retaining wall?

Not recommended. Pea gravel has less void space than #57, so water moves through it slower. In heavy rain, that means hydrostatic pressure can build up before the water drains away. Use #57 washed.

How wide should the gravel column be?

12 inches (1 ft) is the standard minimum for walls under 4 ft tall. Walls 4–6 ft tall typically use an 18-inch column. Anything over 6 ft should be engineered, with column width specified in the design.

Do I need a drain pipe behind a retaining wall?

Yes, in almost all cases. A 4-inch perforated pipe runs along the base of the gravel column and carries the water to a daylight outlet. Without it, the gravel column just delays the water — it doesn't get rid of it.

Do I need fabric behind a retaining wall?

Yes — non-woven geotextile fabric between the gravel column and the soil being retained. Stops fines from migrating into the gravel and clogging the system over years.

How much does retaining wall drainage gravel cost?

For a 30-ft, 4-ft-tall residential wall: about $370 in #57 washed stone, $50 in 4" perforated pipe, and $80 in non-woven geotextile fabric. Total around $500 in drainage materials before the wall blocks themselves.

Can I use crusher run instead of #57 behind a retaining wall?

No — crusher run has too many fines (dust and small particles) and packs too tight to drain properly. Water that can't drain builds up hydrostatic pressure behind the wall, which is the #1 cause of retaining-wall failure. Use clean, washed #57 stone or 3/4-inch washed gravel for the drainage column.

Where does the perforated pipe drain to?

To daylight at the lowest point of the wall, into a dry well, or into the storm drain (where local code permits). Run the pipe with at least 1% slope toward the outlet. The outlet should be lower than every point of the wall, and protected with a rodent-proof grate. Without a working outlet, the gravel column eventually saturates and fails.

Retaining Wall Gravel Calculator

How this calculator works for retaining walls

Behind a retaining wall, you build a vertical column of clean stone that runs the full height of the wall. That column's dimensions are wall length × column width × wall height. The calculator uses these as if it were a rectangle: length × width × depth.

Don't reuse the "depth" field as the column width. Plug your wall length as the calculator's length, your column width (typically 12 inches → 1 ft) as the width, and your wall height as the depth.

Drainage is non-negotiable

A 4-ft tall retaining wall with no drainage column will fail in 5–10 years from hydrostatic pressure alone, regardless of how well the blocks are built. The gravel column is what keeps the wall standing.

At the base of the column, set a 4-inch perforated pipe in a shallow trench, sloped at 1% to a daylight outlet. The column feeds water down to the pipe; the pipe carries it away.

Recommended depth & material

Use	Depth	Material	Notes
Standard segmental wall (under 4 ft)	12" wide column, full wall height	#57 washed crushed stone	Most common DIY-friendly spec.
Tall segmental wall (4 – 6 ft)	18" wide column	#57 washed	Wider column for higher water load. Often requires engineering.
Concrete block / poured wall	12" wide column + weeps	#57 washed	Weep holes through wall every 8 ft.
Timber retaining wall	12" wide column	#57 washed	Drainage even more critical (wood rots).
Boulder wall (low aesthetic)	6" – 12" column	#57 washed	Boulders self-drain somewhat; gravel still helps.

How to measure for retaining-wall gravel

Three numbers: how long the wall runs, how wide the gravel column behind it is, and how tall the wall is.

Measure the wall length. Total linear feet of wall — including any returns at the ends. For a curved wall, measure along the curve, not the chord.
Decide the column width. 12 inches (1 ft) is standard for walls under 4 ft tall. 18 inches for 4–6 ft. Code or your engineer may specify more.
Measure wall height. From the top of the lowest course (which sits buried in the leveling pad) to the top of the wall. A 4-ft visible wall typically has 5 ft of total wall (one course buried).
Plug into the calculator. Set length = wall length (ft), width = column width (ft), depth = wall height (in inches → calculator converts).
Use #57 washed. Default material. Don't substitute pea gravel — it lacks the void space to drain quickly enough during heavy rain.
Add 10–15% buffer. Walls behind soil have irregular back-cut profiles; you'll always use a bit more than the rectangular math suggests.

Worked example: 30-ft long, 4-ft tall retaining wall

A typical residential terrace wall: 30 ft long × 1 ft column × 4 ft tall.

Volume: 30 × 1 × 4 = 120 cuft = 4.44 yd³
Weight: 4.44 × 2,750 ÷ 2,000 = 6.11 tons
With 10% buffer: ~4.9 yd³ / 6.7 tons of #57 washed.
Cost: at $55/ton (washed costs slightly more), ~$370 in stone, plus delivery, plus pipe and fabric.

At 6 ft tall (same wall, taller), the volume jumps to 180 cuft and the order goes to ~10 tons.

Retaining Wall Drainage Gravel Calculator