Calculate materials for a retaining wall including blocks, cap stones, gravel backfill, drainage pipe, and landscape fabric. Supports standard retaining wall blocks from major manufacturers.
Walls over 4 ft typically require engineering
Estimates only — retaining walls over 4 feet should be designed by a licensed engineer. Always check local building codes and obtain permits where required.
Building a 20-foot long, 3-foot tall retaining wall with standard 12×4×16 blocks and 10% waste? You need approximately 149 blocks, 15 cap stones, 2.2 cubic yards of gravel backfill, 24 feet of drainage pipe, and 100 square feet of landscape fabric. At $3–$6 per block, the blocks alone cost $447–$894.
| Wall Size | Blocks (10% waste) | Cap Stones | Gravel Backfill |
|---|---|---|---|
| 10×2 ft | 50 | 8 | 0.7 cy |
| 10×3 ft | 75 | 8 | 1.1 cy |
| 20×3 ft | 149 | 15 | 2.2 cy |
| 20×4 ft | 198 | 15 | 3.0 cy |
| 30×4 ft | 297 | 23 | 4.4 cy |
For standard 12×4×16 blocks, about 2.25 blocks per square foot of wall face. A 20-foot long, 3-foot tall wall is 60 square feet, requiring about 135 blocks plus 10% waste.
Most building codes allow 4 feet maximum without a permit or engineering design. Some areas allow only 3 feet. Any wall holding back a slope near a structure, driveway, or property line usually needs engineering regardless of height.
12 inches of clean gravel backfill, wrapped in landscape fabric to prevent soil from clogging the drainage. A perforated pipe at the base directs water away from the wall. Never backfill with soil — the water pressure will push the wall over.
DIY materials for a basic block wall run $10–$20 per face square foot. Professional installation runs $25–$50 per face square foot depending on height, access, and soil conditions.
Yes, always. Without drainage, water pressure builds behind the wall (hydrostatic pressure) and will eventually push it over. Install a 4-inch perforated pipe at the base, sloped to daylight at one or both ends.
Typical Results
💡 Walls over 3' high typically require geogrid reinforcement per manufacturer specs — consult local codes.
Common Mistakes