How to Calculate Retaining Wall Materials
Work out exactly how many concrete sleepers, steel posts, geofabric and drainage aggregate you need before you order — and avoid costly over-ordering or mid-project shortfalls.
One of the most common questions we get before a retaining wall project is: "How much do I need to order?" Get it wrong and you're either making a second trip to the yard mid-project, or you've over-ordered and wasted money on materials you don't need.
This guide walks you through the calculation for each material component. Or if you'd rather skip the maths, use our free Retaining Wall Estimator — it does all of this automatically.
The Key Variables
Before you can calculate anything, you need to know:
- Wall length — the total run of the wall in metres
- Wall height — the exposed height above ground in metres
- Sleeper thickness — 80mm or 100mm (100mm for walls over 1.2m)
- Post spacing — typically 1.8m or 2.4m centres
Calculating Concrete Sleepers
Concrete sleepers come in standard lengths — most commonly 2.4m, with 1.8m and 3.0m also available.
Number of sleepers per course:
Sleepers per course = Wall length ÷ Sleeper length (round up)
For a 12m wall using 2.4m sleepers: 12 ÷ 2.4 = 5 sleepers per course.
Number of courses:
This depends on sleeper thickness and wall height:
| Sleeper Thickness | Courses per 1m height |
|---|---|
| 80mm | 12.5 courses |
| 100mm | 10 courses |
For a 1.2m wall with 80mm sleepers: 1200 ÷ 80 = 15 courses.
Total sleepers:
Total sleepers = Sleepers per course × Number of courses
For our 12m × 1.2m example with 80mm sleepers: 5 × 15 = 75 sleepers.
Add a 5–10% buffer for cuts, breakage, and waste. Round up to the nearest full pack.
Calculating Steel Posts
Number of posts:
Number of posts = (Wall length ÷ Post spacing) + 1
For a 12m wall at 1.8m post spacing: (12 ÷ 1.8) + 1 = 7.67, round up to 8 posts.
Always add 1 for the end post — a common mistake is to forget the final post at the end of the run.
Post length:
Post length = Exposed wall height + Embedment depth.
The rule of thumb for embedment is 1:1 — embed as deep as the wall is high. So a 1.2m wall needs 1.2m of post in the ground, giving a total post length of 2.4m.
In soft or sandy soils, increase embedment to 1.5× the wall height. In rock or very stiff clay, you may be able to reduce slightly — but always err on the side of caution.
Post type:
| Wall Height | Recommended Post |
|---|---|
| Up to 1.2m | 100 UC H Post or 100 PFC C Post |
| 1.2m – 1.8m | 100 UC H Post |
| 1.8m – 2.5m | 150 UC H Post |
| Over 2.5m | Engineer required |
Calculating Geotextile Fabric
Geotextile fabric (geofabric) lines the back of the wall to prevent soil migrating into the drainage aggregate. You need enough to cover the full height and length of the wall, with an overlap at the top.
Geofabric area = Wall length × (Wall height + 0.5m overlap)
For a 12m × 1.2m wall: 12 × 1.7 = 20.4 m².
FPM stocks geofabric in rolls — check the roll width and length to work out how many rolls you need.
Calculating Drainage Aggregate
A 150–200mm drainage layer behind the wall is standard practice. Calculate the volume of aggregate needed:
Aggregate volume (m³) = Wall length × Drainage layer depth × Wall height
For a 12m wall, 200mm drainage layer, 1.2m high: 12 × 0.2 × 1.2 = 2.88 m³.
Add 10–15% for compaction and wastage. Drainage aggregate is typically sold by the tonne — 20mm blue metal weighs approximately 1.5 tonnes per m³.
Calculating Ag Pipe
Ag pipe (slotted drainage pipe) runs along the base of the wall inside the drainage aggregate. You need one length equal to the wall run, plus enough to reach a drainage outlet.
Ag pipe length = Wall length + outlet run
For a 12m wall with a 3m outlet run: 15m of ag pipe.
Calculating Concrete for Post Footings
Each post footing is typically a 300mm diameter × post embedment depth cylinder.
Volume per footing:
Volume = π × r² × depth = 3.14 × 0.15² × 1.2 = 0.085 m³
For 8 posts: 8 × 0.085 = 0.68 m³ of concrete.
For small quantities, bagged rapid-set concrete is the most practical option. A 20kg bag yields approximately 0.009 m³, so you'd need around 76 bags for this example — or roughly 10 bags per post.
Quick Reference: 12m × 1.2m Wall Example
| Material | Quantity |
|---|---|
| 80mm concrete sleepers (2.4m) | 75 + 10% buffer = 83 |
| 100 UC H posts (2.4m) | 8 |
| Geofabric | 21 m² |
| 20mm drainage aggregate | 3.3 m³ (~5 tonnes) |
| Ag pipe (100mm slotted) | 15m |
| Rapid-set concrete (20kg bags) | ~80 bags |
Use the Free Estimator
If you'd rather not do the maths manually, our free Retaining Wall Estimator handles all of these calculations automatically. Enter your wall length, height, sleeper thickness and post spacing — it outputs a complete materials list with quantities and pricing.
The estimator also covers Colorbond and timber fencing calculations.
Get a Quote
Once you have your quantities, contact FPM Building Supplies for trade pricing. We stock the full range of concrete sleepers, galvanised steel posts, geofabric, ag pipe and rapid-set concrete from our Epping warehouse, with Melbourne-wide delivery available.
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