How Many Blocks for a Retaining Wall?
Calculate retaining wall blocks, cap stones, base gravel, and drainage materials by wall length and height.
Retaining Wall Basics
Retaining walls hold back soil on a slope. Walls under 3 feet are typically DIY-friendly with interlocking blocks. Walls over 4 feet require engineered design in most jurisdictions — the soil pressure increases exponentially with height, and failure of a tall retaining wall can cause property damage or injury.
The Critical Base Layer
The base is the most important part of a retaining wall. Dig a trench 6 inches deep and twice the block width. Fill with 6 inches of compacted crushed stone. The first course of blocks should be half-buried below grade. A level, compacted base prevents settling and leaning — the #1 cause of retaining wall failure.
Drainage Prevents Failure
Water pressure behind a retaining wall is the second most common cause of failure. For walls 2+ feet tall: install a 4-inch perforated drain pipe behind the base course, surrounded by gravel, with landscape fabric separating the gravel from backfill soil. The pipe should daylight at one or both ends to drain water away from the wall.
Interlocking retaining wall blocks have a built-in setback (lip) that angles each course slightly toward the slope. This "batter" (typically 1 inch per course) counteracts the soil pressure pushing outward. Never stack blocks vertically without setback — the wall will eventually topple forward.
