Calculate how many retaining wall blocks you need, plus all materials including gravel, drainage pipe, landscape fabric, and adhesive. Works for standard landscaping blocks, large retaining blocks, Allan Block, and Versa-Lok systems.
The number of blocks depends on wall length, height, and block size. For standard 12x4x8 inch blocks, you need approximately 1 block per linear foot per course. A 20-foot wall that is 2 feet high (6 courses) needs about 120 blocks. Always add 10% for cuts and waste, especially at corners.
Most jurisdictions require permits for retaining walls over 4 feet tall measured from the bottom of the footing to the top of the wall. Walls over 4 feet typically require engineering plans and inspections. Some areas require permits for walls as low as 30 inches. Check with your local building department before starting any retaining wall project.
Plan for a 6-inch deep compacted gravel base that extends 6 inches beyond the wall width on each side. For a 20-foot wall with 12-inch blocks, you need approximately 0.5 tons of crushed gravel (3/4" minus or road base) for the base alone. Drainage gravel behind the wall requires additional material - typically 12 inches wide and the full height of the wall.
Geogrid reinforcement is recommended for walls over 3 feet tall or walls that will retain heavy loads like driveways or structures. The geogrid layers should extend back into the hillside at least 4-5 feet (typically 60-70% of wall height), installed every 2-3 courses of blocks. For walls over 4 feet, an engineer should specify the geogrid spacing and length.
Proper drainage prevents hydrostatic pressure from building up behind the wall, which is the leading cause of retaining wall failure. Water-saturated soil can weigh 50% more than dry soil and creates tremendous pressure. Use drainage gravel behind the wall, perforated pipe at the base to collect water, and landscape fabric to prevent soil from clogging the drainage system.
The base course should be buried at least 1 inch per 8 inches of wall height. For a 2-foot wall, bury 3 inches below grade. For a 4-foot wall, bury at least 6 inches. The gravel base beneath should be 4-6 inches deep and extend 6 inches beyond the block width on each side. This buried portion anchors the wall and prevents frost heaving.