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The length of one straight run of wall, from the centre of the first post to the centre of the last.
Measure along the line the posts will stand on. Run each straight length separately: a corner or a change of direction starts a new run, and so does a step where the wall changes height down a slope. The run is divided into bays at the post centres, and a run that is not a whole number of bays ends in a shorter bay whose sleepers are cut to fit.
Tools needed: Tape measure, String line and pegs
The height of sleepers above the bottom of the lowest course, which is the soil the wall holds back plus any course set below the ground.
The courses are this height divided by the sleeper's face height, rounded up to whole sleepers, so a wall a little over a whole number of courses takes another full one; the result shows how far that top course stands above the height entered, and the posts are taken up to it. The page starts from the one case Outback Sleepers' installation guide works in full, a 1 m (3.28 ft) wall. When the courses stack higher than that, the guide and the maker's specifier page call for an engineer, and the page says so whatever else is entered.
The length of one sleeper as sold, which with the post centres sets how many fit a course.
Outback Sleepers makes its concrete sleepers in 2 m (6.56 ft) and 2.4 m (7.87 ft) lengths and publishes engineering for 1.5 m (4.92 ft) and 1.8 m (5.91 ft) sleepers too; timber sleepers come in the lengths the merchant stocks. The page opens on the maker's 2.0 m (78.74 in) sleeper in both unit systems, because no sleeper for this system is made to an inch size. A sleeper longer than its bay is cut down to it, and the result counts those cut sleepers, so the off-cuts are not a surprise.
How tall one sleeper stands on its edge in the wall, which sets how many courses reach the height.
Measure the face that shows, top edge to bottom edge, as the sleeper sits in the wall. Outback's concrete sleepers are 200 mm (7.87 in) high in every thickness; a timber sleeper's face is its width as sold. Thickness does not enter the count: it is the maker's or the engineer's choice for the wall height, and it decides whether the sleeper spans its bay.
The distance from the centre of one post to the centre of the next, as the sleeper maker or the engineer sets it.
For sleepers that sit between the flanges of H posts, Outback Sleepers sets the centres at the sleeper length plus 10 mm (0.39 in): 2,010 mm (79.1 in) for a 2.0 m (6.56 ft) sleeper and 2,410 mm (94.9 in) for a 2.4 m (7.87 ft) one, so a sleeper drops in with a little play. Its engineering also asks for at least 25 mm (0.98 in) of each sleeper end to bear on its post, so centres much wider than the sleeper are the maker's to approve, not this page's. A timber sleeper may need closer posts than its length, because timber bends more than reinforced concrete; that spacing is the timber supplier's or the engineer's figure.
How far each post goes down into its concrete footing, from the maker's guide for this height or from an engineer.
Each post's length is this plus the height of the courses stacked above it, the post standing to the top of the top course. Outback's guide draws its 1 m (3.28 ft) wall with 1,000 mm (39.4 in) of post in the hole, which an imperial reader opens on rounded up to 40 in (1,016 mm), and its specifier page sets walls 600 mm (23.6 in) high or less at least 600 mm (23.6 in) into the ground. For any other wall, or other ground, the depth is an engineer's, and the maker says the same for every wall over 1 m (3.28 ft).
The diameter of the augered hole each post is concreted into, as the maker or the engineer gives it.
Outback's guide asks for holes at least 450 mm (17.7 in) across for a 1 m (3.28 ft) wall, to be confirmed by a qualified engineer. The concrete in the footing is what stops the post rotating under the soil's push, so the hole is part of the design, and a smaller auger than the figure given is not a saving to make.
Tools needed: Post hole auger or digger
How deep each hole is dug, usually a little deeper than the post goes so concrete closes under the post.
For its 1 m (3.28 ft) wall Outback's guide says the holes must be dug 1,100 mm (43.3 in) deep and draws 100 mm (3.9 in) of concrete under the post, with the hole filled to the ground and not overfilled; an imperial reader opens on that depth rounded up to 44 in (1,118 mm). The concrete is worked out as the full depth of each hole, so enter the depth actually dug.
The weight printed on the bag of pre-mixed concrete you will buy, such as 20 kg (44 lb) or 80 lb (36 kg).
The bags are counted at the site's standard pre-mix yield of 0.0075 cubic feet of concrete for every pound of dry mix (about 0.47 litres a kilogram), the rate the Concrete Bag Calculator uses: about 9.4 litres (0.33 cu ft) from a 20 kg (44 lb) bag and 0.6 cu ft (17 litres) from an 80 lb (36 kg) bag. A rapid-setting post mix, or a bag that prints a different yield, changes the count, so check the bag.
Sleepers for the wall
15 sleepers
Counted the way the maker's own calculator counts a straight wall: one sleeper a course in every bay, a post at every joint, a C section at each end and H sections between. The post section, the centres, the embedment and the hole are the maker's or an engineer's figures for this wall in this ground; the page multiplies them out and does not choose them, and a wall below a slope, a driveway or a building needs an engineer at any height.
- Courses of sleepers
- 5 courses
- Bays between posts, one sleeper a course in each
- 3 bays
- Sleepers cut to fit their bay
- 5 sleepers
- Last bay, post centre to post centre
- 75.73 in
- Height of the stacked courses
- 39.37 in
- Top course above the height entered, posts taken up to it
- 0.37 in
- Steel posts in all
- 4 posts
- End posts, C section
- 2 posts
- Intermediate posts, H section
- 2 posts
- Length of each post, stacked courses plus embedment
- 6.61 ft
- Concrete in each post hole
- 6.48 ft³
- Concrete for all the post holes
- 25.92 ft³
- Bags of pre-mixed concrete
- 44 bags
They open the calculator with your figures already in it
Sleeper Retaining Wall Calculator (Sleepers, Steel Posts and Post Concrete): 15 sleepers — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Outback Sleepers, Installation Guide — Concrete Sleepers: holes at 2,010 mm centres for 2.0 m sleepers and 2,410 mm for 2.4 m sleepers, a similar 10 mm allowance for other lengths; hole diameter at least 450 mm for a one metre high wall, to be confirmed by a qualified engineer; for a one metre wall, post holes dug 1,100 mm deep, drawn with 1,000 mm of post above the ground, 1,000 mm in the hole and 100 mm of concrete under it; for all walls over 1 m, pier depth to be confirmed by a qualified engineer; a C section post drawn at the end of the run and H sections between; a plastic membrane behind the wall, an AG pipe or strip drain at the bottom of the wall covered in 20 mm gravel; no equipment over the backfill within 75% of the wall height; designs for typical soil conditions, and for walls over 1 metre independent engineering advice for the site
- Outback Sleepers, information for specifiers: sleepers in 2 m and 2.4 m lengths, 200 mm high, in a range of thicknesses; walls 600 mm high or less set a minimum of 600 mm into the ground; structures over 1 m in height require council approval and site-specific engineering; steel lengths and sizes to be confirmed by the site engineer
- Outback Sleepers, retaining wall calculator: number of sleepers = (wall height ÷ 0.2) × (wall length ÷ 2), number of steel posts = (wall length ÷ 2) + 1, using the standard 2 m sleeper on a straight wall of constant height; the customer checks product suitability and seeks council approval and engineering advice where required
- Wallbridge Gilbert Aztec for Outback Sleepers, drawing WAD150933-S01 revision 5 (3 July 2020), concrete sleepers, 2,000 mm long: 200 mm high, 75 mm thick for walls up to 2.0 m, 100 mm to 3.0 m and 110 mm to 4.0 m; suitable only where the wall carries no heavy surcharge (no retaining walls, structures or buildings within twice the wall height) and the finished surface above it is flatter than 1 vertical in 10 horizontal; assumed soil 18 kN/m³ and 30°, surcharge 5 kPa; hand compaction only within 0.75 of the wall height; at least 25 mm of end bearing at each end of a sleeper; steel posts and concrete pier footings by others
- Outback Sleepers, steel H-channel posts (NSW): 100 UC 14 galvanised I sections from 600 to 5,000 mm long and 150 UC 23 from 600 to 8,000 mm, hot-dip galvanised, for concrete sleeper retaining walls
- Bag yield: the Concrete Bag Calculator's standard published yield for pre-mixed concrete, 0.0075 cubic ft per pound of dry mix (0.60 cubic ft from an 80 lb bag, about 9.4 litres from a 20 kg bag); the yield printed on the bag governs
Inputs used
- Wall Length
- 19.5 ft
- Wall Height (bottom of the lowest sleeper to the top)
- 39 in
- Sleeper Length
- 78.74 in
- Sleeper Face Height
- 7.87 in
- Post Centres
- 79.13 in
- Post Depth in the Ground (embedment)
- 40 in
- Post Hole Diameter
- 18 in
- Post Hole Depth
- 44 in
- Concrete Bag Weight
- 80 lb
Intermediate steps
- Courses of sleepers
- 5 courses
- Bays between posts, one sleeper a course in each
- 3 bays
- Sleepers cut to fit their bay
- 5 sleepers
- Last bay, post centre to post centre
- 75.73 in
- Height of the stacked courses
- 39.37 in
- Top course above the height entered, posts taken up to it
- 0.37 in
- Steel posts in all
- 4 posts
- End posts, C section
- 2 posts
- Intermediate posts, H section
- 2 posts
- Length of each post, stacked courses plus embedment
- 6.61 ft
- Concrete in each post hole
- 6.48 ft³
- Concrete for all the post holes
- 25.92 ft³
- Bags of pre-mixed concrete
- 44 bags
Confidence note: Counted the way the maker's own calculator counts a straight wall: one sleeper a course in every bay, a post at every joint, a C section at each end and H sections between. The post section, the centres, the embedment and the hole are the maker's or an engineer's figures for this wall in this ground; the page multiplies them out and does not choose them, and a wall below a slope, a driveway or a building needs an engineer at any height.
What this calculation does not cover
- A count of materials, never a design. The post section, the post centres, how deep the posts go, the hole and the concrete are the sleeper maker's published figures for this height in this ground, or an engineer's, and the page multiplies them out. The one case it opens on, Outback Sleepers' 1 m (3.28 ft) wall with holes 450 mm (17.7 in) across and 1,100 mm (43.3 in) deep, is given in the maker's guide with the words 'to be confirmed by a qualified engineer'.
- What the wall holds back matters more than how tall it is. The maker's engineering for its sleepers assumes no heavy surcharge, meaning no existing retaining wall, structure or building within twice the wall height, a finished slope above the wall flatter than 1 vertical to 10 horizontal, and a surcharge of 5 kPa (104 psf). A wall below a slope, beside a driveway or parking, or within twice its height of a house, a shed or a pool needs an engineer at any height.
- Drainage is not counted. Outback's guide puts a plastic membrane behind the sleepers, an agricultural pipe or strip drain along the back of the base and 20 mm (0.79 in) gravel over the pipe, and keeps machines off the backfill within three quarters of the wall height; a wall that holds water is pushed far harder than one that drains.
- One straight run of one height, built one way. Run each straight length on its own and add them: at a corner the two runs each count an end post where the wall takes one corner post, and a wall that steps down a slope changes its courses bay by bay. A wall of sleepers stacked with staggered joints and fixed to one another or to timber posts, or of sleepers stood on end, is not this system and needs a count of its own.
- The concrete is each hole's full cylinder, filled to ground level as the maker's guide draws it. Nothing is taken off for the steel post, a thin section that displaces little, and nothing is added for a hole dug oversize or caving in loose ground. Bags are counted at the site's standard pre-mix yield, rounded up once across the wall; a fast-setting post mix or a different bag can yield otherwise, so read the bag, and for a long wall weigh up ready-mixed concrete instead.
- Timber sleepers bring questions the count cannot answer: whether the timber is treated for in-ground contact, and how far it may span between posts at this height without bending, are the timber supplier's or the engineer's to state. The post centres entered are taken as already settled.
- Approval is a separate question. Outback says structures over 1 m (3.28 ft) need council approval and site-specific engineering; whether a lower wall needs approval is for the council, building control or the building department where it is built to say, so ask before it goes near a boundary, a building or a road.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-10-05 · v1.0.0
Regulatory standards & verification citations6
- Outback Sleepers, Installation Guide — Concrete Sleepers: holes at 2,010 mm centres for 2.0 m sleepers and 2,410 mm for 2.4 m sleepers, a similar 10 mm allowance for other lengths; hole diameter at least 450 mm for a one metre high wall, to be confirmed by a qualified engineer; for a one metre wall, post holes dug 1,100 mm deep, drawn with 1,000 mm of post above the ground, 1,000 mm in the hole and 100 mm of concrete under it; for all walls over 1 m, pier depth to be confirmed by a qualified engineer; a C section post drawn at the end of the run and H sections between; a plastic membrane behind the wall, an AG pipe or strip drain at the bottom of the wall covered in 20 mm gravel; no equipment over the backfill within 75% of the wall height; designs for typical soil conditions, and for walls over 1 metre independent engineering advice for the site
- Outback Sleepers, information for specifiers: sleepers in 2 m and 2.4 m lengths, 200 mm high, in a range of thicknesses; walls 600 mm high or less set a minimum of 600 mm into the ground; structures over 1 m in height require council approval and site-specific engineering; steel lengths and sizes to be confirmed by the site engineer
- Outback Sleepers, retaining wall calculator: number of sleepers = (wall height ÷ 0.2) × (wall length ÷ 2), number of steel posts = (wall length ÷ 2) + 1, using the standard 2 m sleeper on a straight wall of constant height; the customer checks product suitability and seeks council approval and engineering advice where required
- Wallbridge Gilbert Aztec for Outback Sleepers, drawing WAD150933-S01 revision 5 (3 July 2020), concrete sleepers, 2,000 mm long: 200 mm high, 75 mm thick for walls up to 2.0 m, 100 mm to 3.0 m and 110 mm to 4.0 m; suitable only where the wall carries no heavy surcharge (no retaining walls, structures or buildings within twice the wall height) and the finished surface above it is flatter than 1 vertical in 10 horizontal; assumed soil 18 kN/m³ and 30°, surcharge 5 kPa; hand compaction only within 0.75 of the wall height; at least 25 mm of end bearing at each end of a sleeper; steel posts and concrete pier footings by others
- Outback Sleepers, steel H-channel posts (NSW): 100 UC 14 galvanised I sections from 600 to 5,000 mm long and 150 UC 23 from 600 to 8,000 mm, hot-dip galvanised, for concrete sleeper retaining walls
- Bag yield: the Concrete Bag Calculator's standard published yield for pre-mixed concrete, 0.0075 cubic ft per pound of dry mix (0.60 cubic ft from an 80 lb bag, about 9.4 litres from a 20 kg bag); the yield printed on the bag governs
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