Retaining Wall Block Calculator
Count the blocks in a retaining wall from its length and height and your block’s face size — plus when a wall needs an engineer and a permit.
Calculator
A 20 ft × 3.0 ft wall face (60 sq ft) at 0.50 sq ft per block needs about 120 blocks, plus cap blocks and base gravel. Walls over ~3–4 ft usually need an engineer and a permit — confirm with your building department.
Segmental retaining-wall blocks stack to build a face, and the number you need is driven by the area of that face — length times height — divided by the face area of a single block. This calculator does exactly that and rounds up to whole blocks. A very common block is about 8 in high by 18 in wide, which works out to roughly 0.5 sq ft of face; if your block is a different size, read its face area off the spec sheet and enter it.
The result covers the field blocks that make up the wall face. Cap blocks that finish the top, the buried base course, and the gravel and drainage behind the wall are separate — but the block count is the number that sets the bulk of your order.
Formula
Two steps — face area, then blocks:
wall_face = length_ft × height_ftblocks = ceil( wall_face ÷ block_face_area )
- wall_face — the exposed face of the wall, in square feet.
- block_face_area — the face one block covers (height × width ÷ 144).
Rounded up because you buy whole blocks.
Worked example
A 20 ft long, 3 ft tall wall built with blocks of 0.5 sq ft face:
- Wall face: 20 × 3 = 60 sq ft.
- Divide by the block face: 60 ÷ 0.5 = 120 blocks.
Add a course of cap blocks along the 20 ft top, plus the buried base course, and order gravel for the base and the drainage zone behind the wall.
Background & practice
Know when you need an engineer. Short garden walls are a weekend DIY job, but a wall that is tall or holds back a slope, a driveway or any surcharge is a structural element. As a rule of thumb, walls taller than about 3–4 ft of exposed height — or any wall holding a surcharge — usually need an engineer’s design and a building permit. Confirm the height trigger and the setback rules with your local building department before you start; this tool counts blocks, it does not design a wall.
The base and the buried course matter. A stable wall starts below grade: excavate a level trench, compact a gravel base, and bury the first course roughly one-tenth of the wall height (a course, minimum). That buried course is part of your block count — measure the total face including it, or add a course to the result.
Batter, setback and reinforcement. Segmental blocks are not usually stacked dead vertical: each course steps back a little (the setback or batter) so the wall leans into the slope it retains, which is why the manufacturer’s block determines how the courses lock — pinned, lipped or with connectors. Taller walls also need geogrid, a stiff mesh that tails back into the compacted backfill every few courses to anchor the face into the hillside. Geogrid does not change the block count, but it does change the excavation depth behind the wall and the amount of structural backfill, so plan for it when the design calls for it.
Drainage keeps it standing. Well-built segmental walls have free-draining gravel and a perforated pipe behind them so water pressure never builds up. That is wall drainage, not yard drainage — size the gravel from the manufacturer’s detail. Once you have the block count, put the wall face and your price into the retaining wall cost calculator, and see block & paver coverage for common block faces.
Reference table
Field blocks for common wall sizes at a 0.50 sq ft block face (labeled typical — confirm on the product; walls over ~3–4 ft usually need an engineer):
| Length | Height | Wall face | Blocks |
|---|---|---|---|
| 10 ft | 2 ft | 20 sq ft | 40 |
| 20 ft | 3 ft | 60 sq ft | 120 |
| 30 ft | 4 ft | 120 sq ft | 240 |
| 40 ft | 4 ft | 160 sq ft | 320 |