How the calculator works it out
A retaining wall is taller than the height it retains, because the first course sits below the ground in front. Adbri's rule for level ground is to bury the wall by at least H/20 and never less than enough to cover the base. Austral and Firth ask for at least 100 mm. The calculator buries the larger of 100 mm and H/20, then rounds up to whole courses.
levelling pad = length × 0.6 m × 0.15 m of compacted road base · gravel = length × 0.3 m × ( wall height − 0.2 m ) × 1.45 t/m³
sleepers = courses × ⌈ length ÷ ( sleeper + 10 mm ) ⌉ · posts = bays + 1
The 5 % allowance is what Adbri and Austral recommend for breakage and cuts. The top 200 mm behind the wall is soil rather than gravel, so surface water soaks away instead of pouring straight into the drain.
Common block sizes
| Block | Face (mm) | Per m² | Unreinforced height | Face |
|---|---|---|---|---|
| Adbri Versawall | 400 × 200 | 12.5 | 800 mm + cap | Vertical |
| Austral Heron / Hayman | 390 × 198 | 13 | 500–800 mm by state | 10 mm setback per course |
| Adbri Windsor Stone | 295 × 130 | 26.1 | 1,040 mm | Battered 1:4 |
| National Masonry Gardenwall | 295 × 125 | 26.9 | 1 m in ideal conditions | 25 mm setback per course |
| Firth Compac III (NZ) | 455 × 200 | 11 | 0.8–1.0 m | Vertical or battered |
| Firth Garden Wall (NZ) | 300 × 100 | 33.3 | 0.7 m | Battered only |
"Unreinforced height" is the tallest gravity wall each maker publishes for good conditions: firm ground, no load behind the wall, proper drainage. Taller walls need geogrid reinforcement or no-fines concrete behind the blocks, and an engineer's design.
Drainage: the part that holds the wall up
- Gravel zone. 300 mm of clean 20 mm aggregate directly behind the wall is the common figure in Adbri, Austral and Firth guides. Some sleeper makers accept 200 mm as a minimum.
- Ag pipe. A 100 mm slotted, socked pipe along the base behind the first course, falling at least 1:100. Adbri asks for outlets no more than 20 m apart, or 5 m on wet sites. In NSW, exempt walls must drain to the existing stormwater system.
- Filter fabric. A geotextile between the gravel and the soil stops fine soil clogging the drain. The calculator allows fabric behind and under the gravel with 200 mm laps.
The Australian Geomechanics Society lists water pressure first among the causes of wall failure. Blocked weep holes or missing drainage can cause "sudden and catastrophic failure".
Sleeper walls
Sleeper walls use galvanised steel posts: H-posts between sleepers and C-posts at the ends. Posts go at the sleeper length plus 10 mm, so 2.0 m sleepers mean posts at 2,010 mm centres. Hybuilt's rule for post holes is 450 mm across and as deep as the wall is high, so a 1 m wall needs 1 m holes and 2 m posts. That is what the calculator uses. Engineered tables vary with the load behind the wall. Capital Concrete Sleepers' drawings for a 1 m wall on 2 m sleepers call for a 300 mm hole 1.15 m deep with soil only behind it, and 1.45 m deep with a driveway behind it. Lean the posts back 30–50 mm per metre of height.
Do you need approval?
| Where | No approval up to | Main conditions |
|---|---|---|
| NSW | 600 mm | 1 m from boundaries, 2 m from other walls, drains to stormwater |
| VIC | under 1 m | Not protecting adjoining land or part of other building work |
| QLD | 1 m | No surcharge, 1.5 m from buildings and other walls |
| WA | 0.5 m | Some councils want an engineer at any height |
| SA | 1 m | Total for tiered walls; wall plus fence ≤ 2.1 m |
| TAS | 0.5 / 1.2 m | 0.5 m within 1.5 m of a boundary, 1.2 m elsewhere |
| ACT | 1.2 m (building) | Development approval limits as low as 0.4 m near boundaries |
| New Zealand | 1.5 m | No surcharge: driveway, parking, building or slope above |
These are the state and national limits as at September 2026. Councils, planning overlays, easements and heritage listings can add conditions. The retaining wall approval guide gives the legal instrument for each and explains how height and surcharge are measured.
Retaining wall questions
How many retaining wall blocks do I need per square metre?
12.5 per m² of wall face for 400 × 200 mm blocks, about 26–27 for small 295 mm garden wall blocks, plus 5 % for breakage. Count the buried course too: a wall retaining 600 mm is 800 mm of blocks.
How much gravel goes behind a retaining wall?
A 300 mm wide zone of 20 mm aggregate for the height of the wall, less the top 200 mm of soil. That is about 0.26 t per metre of wall for a wall 800 mm high, at 1.45 t/m³.
How far apart are posts on a sleeper retaining wall?
At the sleeper length plus about 10 mm, so 2,010 mm centres for 2.0 m sleepers and 2,410 mm for 2.4 m sleepers. The posts are hot-dip galvanised steel, set in concrete.
Sources and method
- Adbri Masonry, Segmental retaining wall technical specification (June 2025) and Everything you need to know: retaining walls – block sizes, embedment, drainage, 5 % allowance.
- Austral Masonry, Technical design guide (March 2022) – levelling pad, gravel zone, ag pipe, heights by state.
- Firth, Retaining brochure (2024) and Compac III DIY sheet – NZ blocks, base and drainage.
- Hybuilt, Technical design guide; Sunset Sleepers, Installation guide; Capital Concrete Sleepers, Engineer drawings (May 2024) – posts, holes, footings.
- Australian Geomechanics Society, GeoGuide LR6: Retaining walls – failure causes, engineer above 900 mm.
- Approval limits: see the sources listed in the retaining wall approval guide. Aggregate density 1.4–1.5 t/m³ for 20 mm blue metal (Australian landscape suppliers).
Written by DueRiva · Last reviewed 23 September 2026 · Report an error