Retaining Wall Calculator
How many blocks for that retaining wall — plus caps and backfill?
⚠️ Walls over 4 ft, walls supporting structures or driveways, or sloped-site walls need engineering and often permits — verify with your local building department before building.
How this is calculated
- Wall face area = length × height
- Blocks = ⌈face area ÷ block face coverage⌉ (coverage is stamped on the pallet tag)
- Caps = ⌈wall length ÷ cap width⌉
- Backfill gravel = wall length × backfill width × backfill height, in cubic yards
Common questions
How many blocks for a 20 ft × 4 ft retaining wall?
At 80 sq ft of face and a standard 0.75 sq ft block (6 in × 18 in face), that is 107 blocks — 14 cap stones on top — plus about 1.5 yards of drainage gravel behind it. Blocks per pallet run ~48, so three pallets with a spare.
How high can a block retaining wall be without engineering?
Most manufacturer guides and local codes put the un-engineered DIY limit at 4 feet. Above that, the soil pressure math starts demanding geogrid reinforcement, buried block, drainage design, and a permit — a wall failure is a repair measured in thousands, so the 4 ft line is cheap insurance to respect.
Why does a retaining wall need gravel behind it?
Water is what actually pushes walls over. A 12-inch column of clean stone against the back of the block, with drain tile at the bottom, gives groundwater a path out instead of letting saturated soil push on the wall after every rain. Skipping the gravel and drain is the #1 reason short walls lean within five years.
How deep should the first row of blocks be buried?
Bury the first course: dig down 4-6 inches and set the base block on 4-6 inches of compacted crushed stone. The buried block keeps the wall from sliding forward under pressure — visible height will be ~6 inches less than total height, so buy to the total including the buried course.
Concrete blocks, stone, or wood for a retaining wall?
Interlocking concrete blocks: engineered, DIY-friendly, 50-year life — the default choice and what this calculator assumes. Natural stone/boulder: beautiful, no calculable pattern, skilled-labor territory. Treated timber/6×6: cheapest, but 15-20 year life even done right — fine for low garden beds, a losing bet for anything structural.