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.
Retaining wall calculator — frequently asked questions
How many blocks do I need for a retaining wall?
Divide your wall area (length × height in metres) by the face area of one block. For a standard 390 × 190 mm concrete block, the face area is 0.074 m². A 10 m long × 1 m high wall needs about 135 blocks before waste. The calculator handles standard blocks, sleepers and besser blocks with a 10% waste margin included.
How high can I build a retaining wall without an engineer in Queensland?
In Queensland, retaining walls up to 1 m high generally don't require a building approval or engineer sign-off. Walls between 1 m and 1.5 m may need approval depending on your local council. Any wall over 1.5 m or supporting a surcharge load (like a driveway or building) requires engineering design and building approval.
What is the best material for a retaining wall in Australia?
Concrete segmental blocks (like Baines Rock, Adbri or Keystone) are the most popular for DIY retaining walls — no mortar needed and they handle Queensland's reactive soils well. Hardwood sleepers give a natural look but have a shorter lifespan in wet conditions. Besser blocks suit taller engineered walls and can be rendered for a clean finish.
Do retaining walls need drainage behind them?
Yes — always. Without drainage, water pressure builds up behind the wall and will eventually cause it to fail or lean. A 100 mm agricultural drain (ag pipe) at the base of the wall, wrapped in geofabric and surrounded by 20 mm drainage gravel, is standard practice. In Queensland's heavy rain events this drainage becomes critical.
How deep should the base course of a retaining wall be buried?
At least one course (typically 200 mm) below finished ground level for walls up to 600 mm high. For taller walls, bury deeper — a general rule is 10% of the total wall height below ground. This buried course provides the footing stability that stops the wall from sliding or kicking out at the base.