Free tool
Retaining wall calculator
Wall length and height in, sleepers, posts and concrete out.
A sleeper wall is one of the few landscaping jobs where the material count is simple and the consequences of getting the rest wrong are severe. This handles the count and then tells you the two things that actually matter.
The sum
Courses are the wall height divided by the sleeper height. Sleepers per course are the wall length divided by the sleeper length. Multiply the two.
A 10 metre wall at 900mm high, built from 200mm sleepers 2.4 metres long, is 5 courses of about 4.2 sleepers, so 21 sleepers. Posts at 2.4 metre centres over 10 metres come to 5, counting the one that finishes the run.
Post depth is not a guess
The rule of thumb is a third of the exposed wall height in the ground, plus a bit of clearance under the post for the concrete to bear on. A 900mm wall wants roughly 500mm of embedment, so a post about 1.4 metres long, set in a 300mm diameter hole.
That is a rule of thumb for a low garden wall, not an engineering design. The moment the wall carries anything above it, a driveway, a shed, a pool, or a slope that keeps rising behind it, the loads change completely and the rule stops applying.
Drainage is what makes it last
Water behind a wall is the reason walls fail. Saturated soil weighs far more than drained soil and it pushes far harder. Ag pipe along the base, free draining gravel behind the sleepers, geotextile so the gravel does not silt up, and an outlet that actually daylights somewhere. A wall with no drainage is a wall with a countdown on it.
Height and approval
This is the part worth ringing about before you order anything. In both New South Wales and Victoria a retaining wall over about a metre generally needs approval and an engineered design, and under that height it is often exempt. The exact trigger is set by your council and it is not the same everywhere along the border.
A wall carrying a surcharge load, or one near a boundary or an easement, can need approval at any height. Ten minutes with the council beforehand is the cheapest part of the job, and a wall that has to come out is a total loss rather than a repair.
Common questions
How deep should retaining wall posts go?
As a rule of thumb for a low garden wall, about a third of the exposed height into the ground plus some clearance beneath for the concrete, so a 900mm wall wants roughly 500mm of embedment in a 300mm hole. That is a guide for a simple wall and not an engineered design, and any wall carrying a load above it needs one.
Do I need council approval for a retaining wall?
Usually over about a metre, in both NSW and Victoria, and the exact trigger height is set by your council rather than being the same statewide. Walls near a boundary or carrying a driveway, shed or pool above them can need approval at any height. Ring the council before ordering materials.
How many sleepers do I need for a retaining wall?
Wall height divided by sleeper height gives the courses, and wall length divided by sleeper length gives the sleepers per course. Multiply them. A 10 metre wall at 900mm from 200mm by 2.4 metre sleepers is five courses of about 4.2, so 21 sleepers.
Why does a retaining wall need drainage?
Because saturated soil is much heavier than drained soil and pushes much harder, and water pressure behind a wall is the usual reason walls fail. Ag pipe at the base, free draining gravel behind, geotextile to stop the gravel silting up, and an outlet that actually goes somewhere.