Retaining wall block calculator
Enter the wall’s length and height and the size of the block face. The calculator includes the buried first course most walls need.
Wall blocks
140 blocks
- Cap blocks
- 20 caps
- Courses
- 7 rows of 20 blocks
- Base gravel
- 0.74 cubic yards
- Drainage gravel behind wall
- 2.22 cubic yards (12 in wide)
How it works
Blocks per course are the wall length divided by the block’s face length. Courses are the wall height divided by block height, plus one buried course if you include it. Most block makers call for burying the first course so the wall can’t slide at the base.
Base gravel fills a trench under the first course. Drainage gravel assumes a 12-inch-wide column of clean stone behind the wall for its full height, which keeps water pressure from building up.
Example
With wall length 20 ft, wall height above ground 3 ft, block face length 12 in, block height 6 in, buried first course: yes (recommended), cap length 12 in, base trench width 24 in, base gravel depth 6 in, the answer is 140 blocks.
Tips before you buy
- Taller walls often need a permit, an engineered design and geogrid reinforcement. Check local code before you build anything over about 3–4 feet.
- Compact the base gravel and get the first course perfectly level. Every mistake there shows up on top.
- Put a perforated drain pipe at the bottom of the drainage gravel so water can escape.
- Buy a few extra blocks for cuts at ends and curves.
Common questions
How many blocks for a 20 ft long, 3 ft high wall?
With 12 × 6 inch blocks, 20 per course and 7 courses including one buried, for 140 blocks plus 20 caps.
How deep should a retaining wall base be?
A 6-inch layer of compacted gravel is a common minimum, in a trench wide enough for the block plus room behind it. Follow the block maker’s instructions.
Do retaining walls need drainage?
Yes. Clean gravel behind the wall and a drain pipe at the base keep water pressure from pushing the wall over.