Retaining wall calculator
A segmental block wall under 3 feet high is a landscaping project. Above 4 feet it is a structure that holds back soil with real force behind it. The arithmetic below does not change; the requirement for an engineered design does.
Wall dimensions
How this is calculated
- Courses = ⌈(exposed height in inches ÷ block height)⌉ + buried courses.
- Blocks per course = ⌈(length in inches × 12) ÷ block length⌉. Round up: a wall that ends mid-block needs a cut block.
- Total = courses × blocks per course, then × (1 + breakage).
- Caps = blocks per course, since the cap course follows the same run.
- Base gravel = length × 1 ft wide × 6 in deep, compacted, which is 0.019 cubic yards per foot of wall. Add drainage gravel behind the wall separately.
Worked example
A 30 ft wall, 2 ft exposed, 8 × 12 inch blocks, one buried course:
- Courses = ⌈24 ÷ 8⌉ + 1 = 4 courses
- Blocks per course = ⌈360 ÷ 12⌉ = 30
- Total before breakage = 120; with 5% = 126 blocks
- Caps = 30; base gravel = 30 × 0.019 = 0.56 cu yd, about 15 cu ft
The height where arithmetic stops being enough
Segmental block walls rely on their own weight and the setback of each course to resist the soil behind them. That works up to a point, and the point is lower than most people think:
- Under 3 ft — usually a landscaping project. Many jurisdictions require no permit.
- 3–4 ft — often requires a permit and a designed wall, especially if it carries a surcharge (a driveway or slope above).
- Over 4 ft — engineered design in most jurisdictions, with geogrid reinforcement, a proper footing and drainage.
Two things that push a wall into the engineered category regardless of height: a slope above the wall, and any load on top of it. A 2 ft wall holding back a lawn is not the same as a 2 ft wall holding back a driveway.
Drainage is what actually fails
Most retaining wall failures are water, not blocks. Soil behind the wall saturates, the water pressure adds to the soil pressure, and the wall rotates forward. The fix is a gravel drainage column behind the blocks, filter fabric to keep soil out of the gravel, and a perforated pipe at the base leading somewhere.
None of that is in the block count, and skipping it is how a wall that was correctly calculated falls over in year three.
What no block count can tell you
- It does not design the wall. Battering, geogrid, embedment and footing depth are engineered decisions above 3–4 ft.
- It does not calculate drainage aggregate behind the wall, which is roughly 0.5 cubic yards per foot of wall height.
- It does not handle curves, which increase block count because of the cuts.
- It does not include the cap adhesive or geogrid, which are separate material lines.
Questions people actually ask
How many blocks do I need for a retaining wall?
Face area in square feet divided by the block's face area, plus a course for burial below grade and 5% for breakage. A standard 8 × 12 inch block covers two-thirds of a square foot, so 1.5 blocks per square foot of face.
How high can a retaining wall be without an engineer?
Usually 3 to 4 feet, and lower if there is a slope or a driveway above it. Above 4 feet most jurisdictions require a designed wall with geogrid and a proper drainage system.
Do I need gravel behind a retaining wall?
Yes. A gravel column with filter fabric and a perforated drain at the base is what prevents the water pressure that causes most wall failures. It is roughly half a cubic yard per foot of wall height.
Why does my retaining wall lean forward?
Water. Saturated soil behind the wall adds hydrostatic pressure the wall was not designed for. The fix is drainage, not a bigger block.