What this calculator does
Block count is rows times columns. A 6 metre wall 0.6 metres high in 400 by 200 mm blocks takes 3 courses of 15, which is 45 blocks, or 48 with a 5% waste allowance.
What the count does not tell you is whether the wall is safe. Retaining walls fail by overturning or sliding, driven by the soil pressure behind and by water building up in it. Above about a metre most jurisdictions require engineering, and the block count is the easy part of a design that is mostly about drainage and foundation.
The formula
Courses are the wall height divided by the block height, rounded up, and blocks per course are the length divided by the block length, rounded up. Their product plus the waste allowance gives the total. The calculation covers the visible blocks only, not the base course below grade, the drainage aggregate behind, or any geogrid reinforcement.
| Term | Meaning |
|---|---|
| Course | One horizontal row of blocks. |
| Setback | The backward step of each course, which leans the wall into the slope. |
| Geogrid | Reinforcement mesh extending into the backfill, needed on taller walls. |
| Drainage aggregate | Free-draining stone behind the wall, relieving water pressure. Essential and often omitted. |
The inputs explained
| Field | What to enter |
|---|---|
| Wall length (m) | Wall length in metres. |
| Wall height (m) | Exposed wall height. Add the buried base course separately. |
| Block length (cm) | Block length in centimetres. |
| Block height (cm) | Block height. |
| Waste allowance (%) | Waste allowance for cuts and breakage. 5% is usual, more for curves. |
When to use it
Ordering blocks for a garden wall
Blocks come by the pallet, and the count determines how many.
Comparing block sizes
Larger blocks lay faster but are heavier to handle, often 20 kg or more each.
Planning a terraced garden
Two shorter walls are frequently easier and safer than one tall one.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How many blocks for each wall height?
The same wall at a range of heights.
| Wall height | Blocks needed | Rows (courses) | Without waste allowance |
|---|---|---|---|
| 0.4 m | 32 | 2 | 30 blocks |
| 0.6 m | 48 | 3 | 45 blocks |
| 1 m | 79 | 5 | 75 blocks |
Questions
How tall can I build without engineering?
Commonly around a metre, but it varies by jurisdiction and depends on the soil, the slope above and whether anything is loaded behind the wall. Check locally before building. A wall that fails can undermine what it was holding back and injure someone.
Why does drainage matter so much?
Because water building up behind a wall adds hydrostatic pressure that can exceed the soil pressure itself. Free-draining aggregate and a drain at the base relieve it. Poor drainage is the most common cause of retaining wall failure by a wide margin.
Do I need a base course below ground?
Yes. The first course is normally buried, typically about a tenth of the wall height, on a compacted aggregate base. This calculation counts the exposed height only, so add the buried course to the order separately.
What is setback for?
Each course steps back slightly so the wall leans into the slope it retains, which resists overturning. Most segmental block systems build it into the block shape. It also means the wall footprint at the top is behind the base, which affects setting out.
For drainage behind the wall, see the French drain calculator. For brick and block walls, see the brick and block calculator.