The example is 20 feet long, 2 feet out of the ground, 6 inches in the ground, 12 inch blocks that are 6 inches tall. That is 5 courses and 20 blocks each, 100 blocks. Caps are a different unit if the cap is a different length. Base gravel under the first course is a gravel order. A mortared CMU wall is concrete block. A poured footing is concrete.
This count is not a structural design. Tall walls need an engineer. Geogrid is not a block count.
FAQ
How many blocks?
Courses are the exposed height plus the buried depth, divided by the block height. Blocks per course are the length divided by the block length. A 20 foot wall, 24 inches out of the ground, 6 inches buried, with 12 by 6 inch blocks, is 5 courses of 20, 100 blocks.
Is this a cinder block wall?
No. A flat CMU wall uses an 8×16 module and mortar. A segmental retaining wall uses the block the manufacturer sells, often dry-stacked, with a buried course and a gravel base. CMU is the concrete block page.
Sources
- NIST Handbook 44: Inches and feet. Block face size comes from the product, which is why you type it.