A small garden wall of interlocking blocks holds back soil by its own weight. Block makers set a few rules that make it last: part of the first course is buried, the wall sits on a compacted gravel pad, and gravel behind the blocks lets water drain instead of pushing the wall over.
How to use this calculator
- Measure the length of the wall along the ground.
- Enter the height you want above the finished ground. The buried course is added for you.
- Pick a block, or enter the height, face length and depth of the one you plan to buy.
- Choose whether the wall gets cap units on top.
Worked examples
A low garden wall
Wall length: 20 ft · Height above ground: 2 ft · Block: Versa-Lok Standard
- Wall blocks
- 75 blocks
- Wall blocks
- 75
- Cap units
- 15
- Gravel for the base and drainage
- 3.6 tons
- Courses
- 5 courses, 1 buried
- Blocks per course
- 15 blocks
- Leveling pad gravel
- 0.741 cubic yards
- Drainage gravel behind the wall
- 1.85 cubic yards
A curving bed border
Wall length: 30 ft · Height above ground: 1.5 ft · Block: Allan Block Classic
- Wall blocks
- 80 blocks
- Wall blocks
- 80
- Cap units
- 20
- Gravel for the base and drainage
- 5.7 tons
- Courses
- 4 courses, 1 buried
- Blocks per course
- 20 blocks
- Leveling pad gravel
- 1.11 cubic yards
- Drainage gravel behind the wall
- 2.96 cubic yards
A taller wall that needs a pipe
Wall length: 40 ft · Height above ground: 3.5 ft · Block: Allan Block Classic
- Wall blocks
- 189 blocks
- Wall blocks
- 189
- Cap units
- 27
- Gravel for the base and drainage
- 11.8 tons
- Courses
- 7 courses, 1 buried
- Blocks per course
- 27 blocks
- Leveling pad gravel
- 1.48 cubic yards
- Drainage gravel behind the wall
- 6.91 cubic yards
Your own block size
Wall length: 25 ft · Height above ground: 2 ft · Block: Other: enter the block size · Block height: 4 in · Block face length: 12 in · Block depth: 7 in
- Wall blocks
- 175 blocks
- Wall blocks
- 175
- Cap units
- 25
- Gravel for the base and drainage
- 4.3 tons
- Courses
- 7 courses, 1 buried
- Blocks per course
- 25 blocks
- Leveling pad gravel
- 0.926 cubic yards
- Drainage gravel behind the wall
- 2.16 cubic yards
What goes under and behind the blocks
- A leveling pad of compacted gravel: Allan Block asks for a trench 24 inches wide with at least 6 inches of wall rock, and Versa-Lok for a pad about 6 inches thick and 24 inches wide.
- A buried first course: Allan Block buries 1 inch per foot of wall height, and Versa-Lok about a tenth of the exposed height. The calculator always buries at least one full course.
- Drainage gravel about 12 inches deep behind the blocks, all the way up the wall, so rain drains down instead of building up pressure.
- A perforated drain pipe at the back of the base for taller walls: Allan Block asks for one above 4 ft, Versa-Lok above 3 ft.
How tall can a block wall be?
Versa-Lok gives 4 ft as the tallest stable wall without reinforcement, in the best soil and with nothing on top. Taller walls need geogrid layers in the soil behind them, laid to an engineer's design.
The model building code requires a design for any retaining wall that holds back more than 48 inches of soil, and many towns require a permit for it. A slope, a driveway or a building above the wall adds load, so keep those walls lower or have them designed.
Common questions
How many blocks do I need for a retaining wall?
Divide the wall length by the face length of one block for the blocks in each course, and multiply by the number of courses, counting the buried one. The calculator works this out for your block.
How deep should the base of a retaining wall be?
Deep enough for a compacted gravel pad plus the buried course. Allan Block asks for at least 6 inches of gravel, plus 1 inch per foot of wall height buried.
Do I need a permit for a retaining wall?
Often not for a low garden wall, but the model code requires a design for any wall holding back more than 48 inches of soil, and towns set their own rules. Check with your building department.
What gravel goes behind a retaining wall?
Clean crushed stone, often called wall rock or drainage aggregate, filled about 12 inches deep behind the blocks. It lets water drain away instead of pushing on the wall.