Order a few extra blocks (5% is typical for straight walls, more for curves and corners) so you have spares from the same production run.
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 many 12-inch blocks are in a 20 ft course?
240 inches ÷ 12 = 20 blocks per course.
How many blocks per square foot of wall face?
A 12 × 4 inch face is 1/3 sq ft, so 3 blocks per sq ft. A 16 × 6 is 2/3 sq ft (1.5 per sq ft) and an 18 × 8 is 1 sq ft.
How many extra blocks should I buy?
About 5% for a straight wall and 10% for curves, corners or steps.
Is RetainingWallCalc free?
Yes, with no sign-up or ads.
Are my measurements saved?
Yes, in this browser only.