Trade & construction Australia
Retaining Wall Calculator Australia (Sleepers)
A timber sleeper retaining wall is built from horizontal sleepers held between posts set in the ground. This calculator counts the courses and sleepers from the wall's length and height, the posts and their buried length, and the gravel for drainage behind it. It is a materials estimate, not a structural design.
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Usually the same as the sleeper length.
A rule of thumb. Your engineer or supplier will specify the real depth.
Sleepers needed
—Courses
—
Sleepers per course
—
Posts
—
Post length each
—
Drainage gravel
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The working
Allow for your own site. A materials estimate only, not a structural design. Walls above about one metre, near boundaries or retaining a load usually need an engineer and council approval, and the rules differ by state and council.
How many sleepers do I need for a retaining wall?
Divide the wall height by the sleeper height to get the courses, and the wall length by the sleeper length to get the sleepers in each course. A 9.6 m long, 0.8 m high wall of 2.4 m × 200 mm sleepers is 4 courses of 4 sleepers, or 16 in all, with 5 posts.
When approval is needed
Walls above about one metre, walls near boundaries or buildings, and any wall retaining a surcharge such as a driveway or a slope are commonly regulated. Each council and state sets its own rules, and exempt or complying development limits differ. Ask your council or a certifier before you start.
A wall that fails can damage neighbouring property and put you at legal risk, which is why the design matters more than the materials list.
Drainage
Water pressure is the usual cause of failure. A free-draining gravel layer behind the wall, a slotted drain pipe at the base that outlets away from the wall and a filter fabric stop the water building up. The gravel figure here is for that layer; a drain pipe is extra.
To estimate the gravel, use the gravel calculator.
How this is calculated
The rule, step by step, so you can check any figure by hand.
courses = wall height ÷ sleeper height, rounded upsleepers per course = wall length ÷ sleeper length, rounded up; total = courses × per courseposts = wall length ÷ post spacing + 1, rounded uppost length = wall height + buried length (a share of the height you set)gravel volume = length × height × thickness
This estimates materials and is not a design. A retaining wall holds back soil that weighs well over a tonne per cubic metre, and walls over a certain height, or supporting a slope with a building, road or pool above it, generally need engineering and council approval. The threshold varies by state and council, so check before you build.
The buried-post rule of thumb is a starting figure only. The right depth depends on soil, slope, drainage and what loads the wall carries, and an engineer or the supplier's span tables will give it.
A worked example
A 9.6 m long wall, 0.8 m high, built from 2.4 m × 200 mm sleepers.
- Courses, 0.8 ÷ 0.24
- Sleepers per course, 9.6 ÷ 2.44
- Total sleepers16
- Posts5
- Post length, 0.8 + 60% buried1.28 m
- Drainage gravel, 9.6 × 0.8 × 0.32.30 m³
16 sleepers and 5 posts, plus about 2.3 m³ of gravel.
A taller or sloped wall needs a design from an engineer.
Choosing sleepers
Treated pine sleepers are the cheapest, hardwood lasts longer and concrete sleepers are heavier but need no replacement. Check the treatment class is suitable for ground contact, because an unsuitable class rots within a few years.
What it leaves out
Footing concrete, fixings, drain pipe, geofabric and labour are not included. The post depth rule of thumb is not an engineering design and should not be used to certify a wall.
What this calculator does not do
- A materials estimate, not a structural design
- Straight, level walls; sloped sites need more
- Buried post depth is a rule of thumb
- Approval rules vary by council and state
Common questions
How many sleepers for a retaining wall?
Courses (wall height ÷ sleeper height) times sleepers per course (wall length ÷ sleeper length). A 9.6 m wall 0.8 m high needs 16 sleepers of 2.4 m × 200 mm.
How deep should the posts be buried?
It depends on the wall, soil and loads. A common starting point is about half to the full retained height, but follow your engineer's or supplier's specification.
Do I need approval for a retaining wall?
Often, if it is over about one metre or retains a load. Rules vary by council and state, so check first.
Do I need drainage?
Yes. Gravel and a drain pipe behind the wall stop water pressure building up.
What sleeper should I use?
Treated pine, hardwood or concrete. Check the treatment is suitable for ground contact.
Where these numbers come from
- National Construction Code (ABCB) The code that sets minimum building requirements in Australia
- Your Home — Australian Government guide to sustainable homes Independent guidance on insulation, materials and design
Method last reviewed 2026-10-28 This calculator uses fixed rules rather than rates that change, and its method is re-checked against the sources above at least once a year. Out of date? Tell us and we will fix it.
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