How many blocks for a retaining wall?
Block walls are a simple grid — so many blocks across, so many courses up:
Blocks = blocks per course × number of courses
Blocks per course is the wall length divided by the block's face length; the number of courses is the wall height divided by the block height. We round each up to a whole block, multiply them, and add one extra row of caps for the top. A 20 ft wall, 3 ft tall, in 12 in × 4 in blocks comes to 20 × 9 = 180 blocks plus about 20 caps.
Common block sizes
| Block type | Typical face × height |
|---|---|
| Garden / planter wall | 12 in × 4 in |
| Standard segmental (SRW) | 16–18 in × 6–8 in |
| Cap unit | Matches face, ~3–4 in high |
Retaining wall materials
- Segmental concrete block (SRW). Interlocking, mortar-free units — the most popular and DIY-friendly choice.
- Poured concrete. The strongest option for tall or load-bearing walls, but it needs forms, rebar, and a footing.
- Natural stone. Dry-stacked or mortared for a classic look; slower to build and less uniform.
- Timber. Pressure-treated landscape ties are inexpensive but have the shortest life (~15–20 yrs).
- Gabion. Wire baskets filled with rock — free-draining and good for a rugged, modern look.
When you need engineering
Most areas let you DIY a segmental wall up to about 3–4 ft. Above that — or when there's a surcharge (a driveway, slope, or structure pushing on the wall from above) — you usually need geogrid soil reinforcement, an engineer's design, and a permit. Terracing two shorter walls instead of one tall one can keep a project in DIY territory; check your local code first.
How much does a retaining wall cost?
Blocks run about $3–$8 each, and a finished segmental wall is often quoted by the square foot of wall face. The base gravel, drainage stone, pipe, and (on taller walls) geogrid add up, so material is well beyond just the block count — and engineered or tall walls cost considerably more installed.
Build it to last
- Start with a solid base. Compact about 6 in of gravel in a level trench, and bury the bottom course so roughly the first course is below grade.
- Drain behind it. Backfill with drainage gravel and run a perforated pipe — water pressure is what topples walls.
- Step it back. Most blocks have a lip or pins that set each course back slightly (batter) so the wall leans into the slope.
- Know your limit. Walls over ~3–4 ft usually need geogrid, engineering, and a permit. Check local code first.