Retaining Wall Block Calculator
Estimate a segmental retaining wall. Enter the wall length and height and your block face size, and this calculator returns the wall blocks, the cap blocks, and the base gravel in tons.
How to use this calculator
- Enter the wall length and the finished height above grade, in feet.
- Choose the block face width and height from your product (a 12 in × 6 in face is very common).
- The base gravel assumes a leveling trench about 1 ft wide and 6 in deep along the wall.
- Walls over about 3–4 ft usually need engineering, geogrid reinforcement, and a permit — check locally.
The formula
A block wall is a grid: blocks across each course, stacked in courses.
Blocks per course = wall length (in) ÷ block face width. Courses = wall height (in) ÷ block height.
Total blocks = blocks per course × courses. Cap blocks = one course along the top. Base gravel ≈ length × 1 ft × 0.5 ft ÷ 27 × 1.4 tons.
Worked example
A 20 ft long, 3 ft high wall with 12 in × 6 in blocks.
Per course = (20 × 12) ÷ 12 = 20 blocksCourses = (3 × 12) ÷ 6 = 6 coursesTotal = 20 × 6 = 120 blocksCaps = 20
A 20 ft × 3 ft wall needs about 120 blocks plus 20 cap blocks.
Frequently asked questions
How many retaining wall blocks do I need?
Multiply blocks per course (wall length ÷ block width) by the number of courses (wall height ÷ block height). A 20 ft × 3 ft wall with 12×6 in blocks needs about 120 blocks.
How deep should the base be for a retaining wall?
Dig a leveling trench and bury the first course about 10% of the wall height (at least one full block). Set it on 4–6 inches of compacted gravel base.
When does a retaining wall need engineering?
Walls taller than about 3–4 feet, walls holding back a slope or surcharge, and tiered walls usually require an engineer, geogrid reinforcement, and a permit. Always check local codes.