How this calculator works
Courses are the exposed height divided by block height, plus any buried courses. Blocks per course are the wall length divided by block face length. Drainage gravel fills a zone behind the full wall height.
Courses = round up( Height ÷ Block height ) + Buried courses
Blocks = round up( Courses × round up(Length×12 ÷ Block length) × (1 + Waste%/100) )
Caps = round up( Length×12 ÷ Cap length )
Gravel = Length × Wall height × Drain depth ÷ 27 (yd³)
Assumptions and tips
- Straight wall. Curves and corners change counts; follow the block maker’s design charts for setback, base and reinforcement.
- Leveling-pad base gravel under the first course is not included.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Wall length | 20 ft |
| Exposed height | 24 in |
| Block face length | 16 in |
| Block height | 6 in |
| Buried courses | 1 |
| Cap length | 16 in |
| Drainage gravel depth behind wall | 12 in |
| Waste allowance | 5 % |
| Result | Value |
|---|---|
| Wall blocks | 79 blocks |
| Cap stones | 15 caps |
| Courses (incl. buried) | 5 |
| Blocks per course | 15 |
| Drainage gravel | 1.85 cubic yards |
Step by step:
- Courses = 24 ÷ 6 → 4 + 1 buried = 5
- Blocks per course = 240 ÷ 16 → 15
- Blocks = 5 × 15 × 1.05 → 79
Blocks for common garden walls
16 × 6 in blocks, one buried course, 5% waste.
| Wall | Wall blocks | Cap stones |
|---|---|---|
| 10 ft × 12 in | 26 blocks | 8 caps |
| 20 ft × 18 in | 63 blocks | 15 caps |
| 20 ft × 24 in | 79 blocks | 15 caps |
| 30 ft × 30 in | 145 blocks | 23 caps |
Common mistakes to avoid
- No drainage gravel or pipe behind the wall — water pressure is the top cause of failure.
- Building on soft soil without a compacted leveling pad.
Frequently asked questions
How many retaining wall blocks do I need?
A 20 ft wall, 24 in exposed with 6 in blocks and one buried course needs 5 courses of 15 blocks — 75 blocks, about 79 with 5% waste.