Retaining Wall Calculator — Blocks, Caps & Gravel
Enter your wall length and height and pick a block size to get the number of blocks and courses, plus cap blocks, gravel base, and drainage backfill.
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How it works
blocks = ceil(length×12 ÷ block_width) × ceil(height×12 ÷ block_height)
Blocks by wall size (12 × 4 in garden-wall block)
| Wall (length × height) | Courses | Blocks | Cap blocks |
|---|---|---|---|
| 10 × 2 ft | 6 | 60 | 10 |
| 20 × 2 ft | 6 | 120 | 20 |
| 20 × 3 ft | 9 | 180 | 20 |
| 30 × 3 ft | 9 | 270 | 30 |
| 40 × 4 ft | 12 | 480 | 40 |
For the common 12-in-wide × 4-in-high garden-wall block; larger blocks need fewer units (a 16×8-in block covers about 2.7× the face). Bury the bottom course below grade and add 5–10% for cuts. Walls over 3–4 ft usually need engineering and a permit.
Frequently asked questions
How many retaining wall blocks do I need?
Multiply blocks per course (wall length ÷ block width) by the number of courses (wall height ÷ block height). For example, a 20-ft-long, 3-ft-high wall in 12×4-inch blocks needs 20 per course × 9 courses = 180 blocks, plus 20 cap blocks.
How much gravel do I need behind a retaining wall?
Allow a 12-inch-deep drainage zone of free-draining gravel behind the wall. For a 20-ft × 3-ft wall that is about 2.2 cubic yards, plus roughly 0.7 cubic yard for a 6-inch leveling pad under the base course. The calculator above figures both.
How high can I build a retaining wall myself?
As a rule of thumb, walls up to about 3–4 feet can be DIY with a proper compacted base and drainage. Taller walls carry much more soil pressure and usually need an engineered design, geogrid reinforcement, and a permit — check local codes.