A Retaining Wall Is Mostly Drainage
The block is what you see; the drainage behind it is what decides whether the wall is still standing in ten years. Saturated soil pushes on a wall far harder than damp soil does, and hydrostatic pressure is the most common reason segmental walls bulge, lean, and eventually fail. That is why this list quotes the drain rock, the perforated pipe, and the filter fabric as line items rather than leaving them as an afterthought.
Every line traces back to its standalone calculator. Block, base, and drain rock come from the Retaining Wall Calculator; the pipe comes from the French Drain Calculator. The rock is deliberately counted once— the drain behind a wall and the rock column are the same stone, and quoting both calculators' rock figures would hand you roughly double what you need.
A Worked Example: 24 ft Wall, 3 ft High
A 24-foot wall three feet high in standard 16 × 6 × 12 in SRW block, capped, with a 6-inch base pad and a 12-inch drain rock column:
- 119 wall blocks — 18 per course across 6 courses, with 10% waste
- 18 cap blocks for the top course
- 1.37 tons of crusher run for the compacted leveling pad
- 4.11 tons of clean drain rock behind the wall
- 26 linear feet of 4-inch perforated pipe at the base of the rock
- 132 square feet of non-woven filter fabric
The drain rock is three times the base gravel here. That ratio surprises people who budgeted for a leveling pad and nothing else, and it only grows with wall height: the base pad depends on wall length alone, while the rock column scales with length and height. Take the same wall to 4 feet and the block goes to 159 while the rock climbs to 5.48 tons — but the base gravel does not move at all.
Two Different Stones, Two Different Jobs
Base gravel is crusher run or dense-grade aggregate — stone plus fines that compacts into a solid, level pad for the first course to sit on. Drain rock is clean washed stone with the fines washed out, because its entire purpose is open void space for water to travel down to the pipe.
Putting crusher run behind the wall because it was already on the truck defeats the drainage completely — it compacts, the voids close, and the wall now has saturated soil pressing on it with nowhere for the water to go. They are separate lines on this list for exactly that reason.
The Details That Decide Whether It Lasts
- Bury the first course. Roughly one-tenth of the wall height below finished grade — about 4 inches on a 3-foot wall. The buried course is what keeps the base from sliding forward.
- Compact the pad, then check it level. Every error in the first course is multiplied by every course above it, and there is no fixing it later.
- Give the pipe somewhere to go. A perforated pipe that dead-ends at both ends is a decorative touch. It needs fall to a daylight outlet or a dry well.
- Wrap the rock, not the soil. Non-woven filter fabric goes between the drain rock and the retained soil so fines cannot migrate into the voids. Woven weed barrier is designed to shed water and is the wrong product here.
When You Need an Engineer
Many jurisdictions require an engineered design and a permit at 4 feet, measured from the bottom of the base rather than from finished grade — and frequently lower where the wall carries a surcharge such as a driveway, a slope, or a structure above it. This page flags the threshold, but your building department sets the real number.
An engineered wall commonly calls for geogrid: soil reinforcement fabric laid back into the retained soil at specified course intervals and lengths. Those figures come out of the engineered design and depend on wall height, soil type, and loading, so this calculator does not guess at them. If your design calls for geogrid, get the schedule from the engineer and add it to this list.
What This List Does Not Include
Masonry adhesive for the cap course is not quoted, because published coverage varies substantially by tube and cap size — check the tube. Also budget separately for geogrid where required, drainage outlet fittings, and any excavation or backfill soil. Bulk stone is delivered by the ton with a haul charge per load, so price delivery alongside the material.
Export Your List
Use the buttons above the article to copy the list, download a PDF, or email it to yourself. When supplier links are configured, the “Send this list to Home Depot” section opens a live, in-stock search for every line item — one click per material.
Related
- Retaining Wall Installation Guide — step-by-step build
- Retaining Wall Calculator — block, base, and rock on their own
- French Drain Installation Guide — how the drain run is built
- Dry Well Installation Guide — where the pipe can outlet
- All project materials lists
Written by Daniel McCarney — AceCalc