Landscaping
Retaining Wall Calculator
Work out block quantity and base requirement for a retaining wall, as a planning check rather than a structural design.
Result
Walls above about four feet, or any wall carrying a driveway, slope or structure above it, normally require engineered design and a permit. This estimates materials only. It is not a structural design.
Get a Free Business AuditWhat moves the number
- Wall height, which determines whether engineering is required.
- Base course depth and the levelling pad, which set the whole wall.
- Backfill and drainage behind the wall, which is what keeps it standing.
Where this one goes wrong
Building a retaining wall without drainage behind it. Retained soil holds water, water adds enormous pressure, and a wall without drainage and proper backfill fails. This is a structural matter, not a finish one.
Before you quote
- Whether the height requires engineered design and a permit, which is set locally.
- Drainage aggregate and pipe behind the wall.
- Whether geogrid reinforcement is required for the height and soil type.
Questions
About this calculator
When does a retaining wall need engineering?
Above a height set by local code, and sooner where there is a surcharge such as a driveway or structure above it. The threshold is local and it is a permit matter rather than a preference.
Why does drainage matter so much?
Because saturated soil exerts far more pressure than drained soil. Most retaining wall failures are drainage failures rather than block or base failures.
Does this calculate structural adequacy?
No. It gives block and base quantities for planning. Whether a wall of a given height in a given soil with a given surcharge is adequate is an engineering question.
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How to use this calculator
- Wall length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Wall height (ft) (ft). Enter your own figure; the page starts at 3 ft.
- Block length (inches) (in). Enter your own figure; the page starts at 12 in.
- Block height (inches) (in). Enter your own figure; the page starts at 8 in.
- Cost per block ($). Enter your own figure; the page starts at $6.4.
- Waste allowance (%). Enter your own figure; the page starts at 8%.
The result updates as you type, so you can try a figure, see what it does to blocks needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Wall length (ft) | 40 ft |
| Wall height (ft) | 3 ft |
| Block length (inches) | 12 in |
| Block height (inches) | 8 in |
| Cost per block | $6.4 |
| Waste allowance | 8% |
What comes out
| Result | Value |
|---|---|
| Blocks needed main result | 216 |
| Courses high | 5 |
| Base gravel (cubic yards) | 1.5 |
| Block cost | $1,382 |
A 40 ft × 3 ft wall is 5 courses and about 216 blocks, costing $1,382, plus roughly 1.5 cubic yards of base gravel.
How sensitive the answer is
Holding everything else steady and changing only cost per block, this is what happens to blocks needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per block ($) | Blocks needed |
|---|---|
| $3.2 | 216 |
| $4.8 | 216 |
| $6.4 example above | 216 |
| $9.6 | 216 |
| $12.8 | 216 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Wall length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Wall height (ft) (ft)
- Measured in ft, between 0 and 20, starting at 3 ft.
- Block length (inches) (in)
- Measured in in, between 0 and 48, starting at 12 in.
- Block height (inches) (in)
- Measured in in, between 0 and 24, starting at 8 in.
- Cost per block ($)
- A money amount, starting at $6.4.
- Waste allowance (%)
- A percentage, between 0 and 40, starting at 8%.
What the results tell you
| Result | What it is |
|---|---|
| Blocks needed | The headline result. A number. |
| Courses high | A number. |
| Base gravel (cubic yards) | A decimal figure. |
| Block cost | A money figure. |
Seasonality is not in this
Coverage and material figures assume the area you enter. Slope, access for machinery, existing growth and the time of year all affect both quantity and labour, and none of them appear in a calculation like this.
Keep going
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