RetainingWallCalc rounds the buried depth up to whole courses, so short blocks may need two courses below grade. Always follow the installation guide for your block system.
How the retaining wall calculator works
RetainingWallCalc divides the wall length (in inches) by the block face length and rounds up to get the blocks per course. It divides the exposed height by the block height for the courses above ground, then adds enough buried courses to reach the bury depth: 1 inch per 8 inches of exposed height, at least 6 inches. Blocks = blocks per course × courses, plus your waste allowance. Caps = wall length ÷ cap length, plus the same allowance.
The base trench is the block depth plus the extra width you set (12 inches gives 6 inches in front and behind), at the base depth, plus a compaction allowance. Drainage stone fills a zone behind the blocks for their full height. Cubic yards are converted to tons with the density you enter.
Typical block sizes
| Block (face × height × depth) | Blocks per sq ft of face | Typical use |
|---|---|---|
| Small garden wall block 12 × 4 × 7 in | 3 | Garden edging, raised beds, walls to about 2 ft |
| Medium retaining block 16 × 6 × 10 in | 1.5 | Landscape walls to about 3 ft |
| Large retaining block 18 × 8 × 12 in | 1 | Retaining walls to about 4 ft without geogrid (check the maker) |
Bury depth by wall height
| Exposed height | Minimum buried block |
|---|---|
| 2 ft (24″) | 6″ |
| 3 ft (36″) | 6″ |
| 4 ft (48″) | 6″ |
| 5 ft (60″) | 7.5″ |
| 6 ft (72″) | 9″ |
Bury depth, 6-inch base and 12-inch drainage zone are common manufacturer guidance for segmental block walls; your block system's installation guide wins. Walls retaining more than 48 inches of soil need an engineered design under the IRC (R404.4), as do walls with slopes, driveways or buildings above them. Results are estimates: confirm quantities with your supplier and call 811 (US) before you dig.
Frequently asked questions
How deep should the first row of blocks be?
1 inch per 8 inches of exposed height, minimum 6 inches. For a 3-foot wall, bury at least 6 inches (one 6-inch block).
How deep is the trench in total?
Buried block plus the base: for a 6-inch block on a 6-inch base, dig about 12 inches below the finished grade at the front of the wall.
When does a retaining wall need an engineer?
The IRC (R404.4) requires a design for walls retaining more than 48 inches of unbalanced fill, and local rules may be stricter, for example with a slope or a driveway above.
Is RetainingWallCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.