How Many Blocks for a Retaining Wall?
Part of Landscaping & Aggregate
Quick answer
Divide the wall height by the block's face height for the number of courses (rows), and the wall length by the block's face width for the blocks per course, then multiply the two. A 20-ft-long, 3-ft-tall wall using 12-inch-wide by 8-inch-tall blocks needs 5 courses of 20 blocks — about 100 blocks — plus a cap row and a buried base course.
A segmental retaining wall is just blocks stacked in offset rows (courses) on a level gravel base. The block count comes from two simple divisions — one for how many rows tall, one for how many blocks long — multiplied together, then nudged up for waste and a cap row.
The formula
- Courses = wall height (in) ÷ block face height (in), rounded up.
- Blocks per course = wall length (ft) ÷ block face width (ft), rounded up.
- Wall blocks = courses × blocks per course, + ~5% waste.
- Cap blocks = wall length ÷ cap length (one finished row on top).
- Add one buried base course below grade for stability.
Blocks for a common wall (12×8 in block)
| Wall length | 1 ft tall | 2 ft tall | 3 ft tall |
|---|---|---|---|
| 10 ft | ~30 | ~40 | ~60 |
| 20 ft | ~60 | ~80 | ~120 |
| 30 ft | ~90 | ~120 | ~180 |
| 40 ft | ~120 | ~160 | ~240 |
Counts above include a buried base course. The retaining wall calculator does the exact math for your block size and adds the cap blocks and gravel base.
FAQs
How many retaining wall blocks are in a square foot?
It depends on the block face. A 12×8 inch block has a face of 0.67 ft², so you need about 1.5 blocks per square foot of wall. A larger 18×6 inch block covers 0.75 ft², or about 1.3 blocks per square foot.
Do I include the buried course in my block count?
Yes — burying the bottom course below grade is standard practice for stability, and those blocks still have to be bought. The calculator adds one base course automatically when that option is on.