How the retaining wall calculator works
The calculator works out the number of block courses by dividing the wall height by the block height (including a standard 10 mm mortar joint). It then calculates blocks per course by dividing wall length by block length (also including a 10 mm joint). Total blocks = courses × blocks per course, with a standard 10% waste margin for cuts and breakage.
For sleeper walls, the calculator works out the number of sleepers needed per course and total courses — sleepers are typically stacked horizontally with no mortar.
Drainage — the most important part of any retaining wall
Poor drainage is the number one cause of retaining wall failure in Australia. Every retaining wall needs a drainage aggregate layer behind it (coarse gravel or drainage stone), plus a slotted agricultural drain pipe (ag pipe) at the base. Water trapped behind a wall generates enormous hydrostatic pressure — enough to push the wall over.
Standard practice is a 300 mm minimum drainage aggregate zone behind the wall. This is calculated as: wall length × wall height × 0.3 m depth. The calculator uses this figure for the drainage aggregate quantity. You'll also need class 3 or class 4 geotextile fabric to line the drainage zone.
Footing requirements for AU retaining walls
- Walls ≤ 400 mm — a well-compacted 100 mm gravel base is typically sufficient for garden landscape blocks.
- Walls 400 mm – 1.0 m — a concrete strip footing is standard. Typical size is 400 mm wide × 200 mm deep for a single-skin wall.
- Walls over 1.0 m — engineer certification and a building permit are required in most Australian states. Do not proceed without engineering advice.
The 1.0 m rule is codified under Australian Building Code provisions and state planning regulations. Even if your wall looks fine without engineering, councils can order removal of non-compliant walls.
Do I need a permit for a retaining wall?
In Queensland, walls over 1.0 m require building approval. In NSW and Victoria, the threshold varies by council but is generally 1.0 m for residential properties. In WA, walls over 0.5 m affecting drainage or neighbouring properties may require approval. Always check with your local council before starting construction.