Enter the wall length, the height above ground and the block size. The calculator adds the buried part of the wall, counts the blocks and caps, and estimates the base gravel and the drainage stone behind the wall.
Retaining wall formulas
Buried depth = 1 in for every 1 ft of wall height Courses = (exposed height + buried depth) ÷ block height, rounded up Blocks per course = wall length ÷ block length, rounded up Caps = blocks per course Base gravel = length × trench width × base depth Drainage stone = length × drainage zone width × wall height
Allan Block’s installation guide calls for at least 6 in of wall rock in the base trench, and a trench deeper by 1 in for each foot of wall height, which is the buried part of the first course.
Worked example: 30 ft long, 3 ft high, 18 × 8 in block
- Buried depth: 3 in; total height 39 in.
- Courses: 39 ÷ 8 = 4.9, so 5.
- Blocks per course: 360 ÷ 18 = 20; total 100 blocks plus 20 caps.
- Base gravel: 30 × 2 × 0.5 ft = 1.11 cu yd. Drainage stone 12 in wide: 3.33 cu yd.
Important limits
Walls taller than about 3 to 4 ft, walls holding back a slope or driveway, and walls near buildings often need geogrid reinforcement and an engineered design, and many places require a permit. Check the block maker’s height chart and your local rules before building.
Questions
How many blocks for a 50 ft wall, 2 ft high?
136 blocks of 18 × 8 in, including the buried course.
Do I need drainage stone?
Yes. Free-draining stone behind the wall relieves water pressure; many designs also add a drain pipe at the base.
Why bury the first course?
It locks the base of the wall in place.
Sources
- How to Install a Retaining Wall, Allan Block.
Used for: Minimum 6 in of wall rock in the base and 1 in of buried depth per 1 ft of wall height.