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 |
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.