Retaining wall calculator
Blocks, caps and tons of gravel for a block retaining wall, with the buried course and base trench included.
Your wall takes
105 blocks
- Caps
- 21
- Courses
- 5 × 20 blocks
- Buried below grade
- 3 in
- Base trench
- 24 in wide × 9 in deep
- Wall rock
- 6.5 tons (4.81 yd³)
- Drain pipe
- 0 ft
- Estimated materials
- Over 6 ft is past the residential install guide: get an engineered design with geogrid.
Following the block maker's residential install guide. Wall rock covers the base and 12 in behind the blocks, not the block cores. Many areas need a permit or an engineer for taller walls and for any wall holding up a driveway or slope; check local code.
How to figure retaining wall materials
Part of the wall goes below grade, so count courses for the exposed height plus the buried part, then add the base trench and the wall rock behind the blocks. The rules follow the Allan Block Residential Installation Manual.
- Buried block = 1 in for each foot of wall height
- Courses = (height + buried) ÷ block face height, rounded up; blocks = courses × (length ÷ block face length, rounded up), plus extra
- Caps = length ÷ cap length, plus extra
- Base trench = 24 in wide, 6 in + the buried depth deep, with 6 in of wall rock
- Wall rock = the trench layer + 12 in behind the blocks for the full built height, in cubic yards and tons
Worked example
Your 30 ft wall, 3.0 ft high, buries 3 in of block, so it's 5 courses of 8 in blocks and 20 blocks per course: 105 blocks with 5% extra and 21 caps. Dig the trench 24 in wide and 9 in deep. The base layer and the rock behind the blocks come to 4.81 cubic yards, about 6.5 tons of clean crushed stone.
Materials per 10 ft of wall
With the default block (18 × 8 in face, about 1.0 sq ft each), capped, no extra. Wall rock at 2,690 lb per cubic yard (crushed stone).
| Height | Courses | Blocks | Caps | Wall rock |
|---|---|---|---|---|
| 1 ft | 2 | 14 | 7 | 1.2 t |
| 2 ft | 4 | 28 | 7 | 1.8 t |
| 3 ft | 5 | 35 | 7 | 2.2 t |
| 4 ft | 7 | 49 | 7 | 2.8 t |
| 5 ft | 9 | 63 | 7 | 3.5 t |
| 6 ft | 10 | 70 | 7 | 3.8 t |
Retaining wall questions
How many retaining wall blocks do I need?
Divide the wall length by the block's face length for blocks per course, and the height plus the buried part by the block's face height for courses. With 18 × 8 in blocks, a 10 ft wall 3 ft high is 5 courses of 7: 35 blocks before extra. Check your block's face size; makers list it on the spec sheet.
How much gravel do I need for a retaining wall?
Enough for a 6 in layer in a 24 in wide trench and a 12 in wide column behind the blocks for the full height. For a 10 ft wall 3 ft high that's about 2.2 tons of clean crushed stone (1.60 yd³), plus what fills the block cores.
How deep should a retaining wall be buried?
The install guide buries 1 in of block for every foot of wall height, on top of 6 in of compacted wall rock, so a 4 ft wall's trench is 10 in deep. Frost, soil and slope can call for more; follow the maker's guide and local code.
Does a retaining wall need a drain pipe?
The guide calls for a drain pipe behind gravity walls over 4 ft tall, any wall with geogrid, and sites with poor drainage. The calculator adds one along the full wall once it's taller than 4 ft.
How tall can a block retaining wall be without reinforcement?
It depends on the block, the soil and what's above the wall. The residential guide covers walls under 6 ft; taller walls, or walls holding up a driveway, building or steep slope, need geogrid and an engineered design. Many areas also require a permit for shorter walls, so check with your building department.