Key Takeaways
- Wall height divided by block height gives the number of courses
- Wall length divided by block width gives the blocks needed per course
- Multiplying the two gives the structural block count
- An optional cap row adds one more row of blocks along the top
- This is a straight vertical block count, not a full wall design
A segmental retaining wall is built up in courses, or rows, so counting the blocks it needs comes down to two divisions and a multiplication. Get the wall’s height and length, and the block’s height and width, and the count follows directly from those four numbers.
Courses come from height, blocks per course come from length
The wall height divided by the block height, rounded up, gives the number of courses. A 24 inch wall built from 6 inch blocks needs 4 courses, because 24 divided by 6 is exactly 4, and any height that does not divide evenly still rounds up to a full course. The wall length, converted to inches and divided by the block width, rounded up, gives how many blocks fill one course. A 20 foot wall built from 18 inch wide blocks needs 14 blocks per course, since 240 inches divided by 18 rounds up from 13.3. Multiplying courses by blocks per course gives the structural block count, 56 blocks for the example above, before any waste allowance or cap row.
A cap row is a separate line, not part of the structural count
Many segmental wall systems finish the top course with a cap block, a flatter unit than the structural blocks below it, laid one row deep along the full wall length. When a cap row is included, it adds one more row’s worth of blocks, using the same blocks-per-course figure as the structural courses, rather than folding into the course count itself.
What a straight count does not cover
This is a vertical block count for a straight wall face, and it stops there deliberately. It does not account for setback, the slight backward step each course of a battered wall takes from the one below it, which shortens the effective length of the upper courses as the wall gets taller. It does not include base gravel, drainage behind the wall, or the geogrid reinforcement that taller segmental walls typically need to stay stable. None of that is optional once a wall gets beyond a modest garden height: check the block manufacturer’s installation guide and your local building code for what a wall of your height actually requires before ordering material or breaking ground. A short retaining wall is a reasonable do-it-yourself project; anything taller starts to carry real engineering requirements that a block count alone will not satisfy.
Get your own course and block count from the retaining wall block calculator. For the base beneath the wall, the gravel calculator and aggregate calculator handle the compacted base layer and drainage stone separately, and if the wall ties into a concrete footing or cap, the concrete calculator works out that volume on its own.