Why Getting Your Quantities Right Matters

Ordering too little means delays and extra freight costs. Ordering too much wastes money. Getting your material quantities right before you order saves time, money, and frustration. This guide walks you through the calculation for every component of a concrete sleeper retaining wall.

What You Need to Know Before You Calculate

Have these measurements ready:

• Wall length — total lineal metres of wall (measure along the base)
• Wall height — finished height above ground level in metres
• Sleeper thickness — 75mm or 100mm (see our 75mm vs 100mm guide)
• Post spacing — typically 1.8m–2.0m centres for standard residential walls

Step 1 — Calculate Number of Sleepers

All our concrete sleepers are 2000mm (2.0m) long and 200mm wide.

Formula:

• Sleepers per vertical row = Wall length (m) ÷ 2.0m (sleeper length) — round up to nearest whole number
• Number of rows = Wall height (m) ÷ 0.2m (sleeper width) — round up to nearest whole number
• Total sleepers = Sleepers per row x Number of rows
• Add 5–10% for cuts, waste, and breakage

Worked example — 10m long wall, 1.0m high:

• Sleepers per row = 10 ÷ 2.0 = 5 sleepers per row
• Number of rows = 1.0 ÷ 0.2 = 5 rows
• Total sleepers = 5 x 5 = 25 sleepers
• Add 10% waste = 28 sleepers (order 30 for safety)

Step 2 — Calculate Number of Steel Posts

Standard post spacing is 1.8m–2.0m centres. Use 1.8m spacing for taller walls or reactive soils; 2.0m for standard residential walls under 1m.

Formula:

• Number of posts = (Wall length ÷ Post spacing) + 1
• Always add 1 for the end post
• Add corner posts, L-posts, and end caps as required for your wall layout

Worked example — 10m wall at 2.0m spacing:

• Posts = (10 ÷ 2.0) + 1 = 6 posts

Post length: Total post length = finished wall height + embedment depth. Embedment = at least 1/3 of total wall height, minimum 600mm.

• Example: 1.0m wall + 600mm embedment = 1.6m minimum post length. Order 1.8m or 2.1m posts to allow for variation in ground level.

See our full steel post range for available lengths, or our H-Beam vs C-Channel guide for post type selection.

Step 3 — Calculate Concrete for Post Footings

Each post hole requires concrete to secure the footing. As a guide:

• Post hole diameter: 250–300mm
• Post hole depth: embedment depth + 100mm (gravel base)
• Concrete volume per hole: approximately 0.015–0.025m³ (15–25 litres) for a standard 250mm diameter hole
• Use 20MPa minimum concrete mix

Worked example — 6 posts, 700mm embedment depth:

• Hole volume = π x (0.125)2 x 0.7 = ~0.034m³ per hole
• Total concrete = 6 x 0.034 = ~0.2m³ (approximately 8 x 20kg bags per hole, or order 0.25m³ premix)

Step 4 — Calculate Drainage Materials

Every retaining wall needs drainage. See our full drainage installation guide for details.

Ag pipe: Order the same length as your wall + 10% for outlet runs. For a 10m wall, order 11–12m of ag pipe.

Drainage gravel: Volume = Wall length x 0.3m (width of gravel zone) x Wall height. For a 10m x 1.0m wall: 10 x 0.3 x 1.0 = 3.0m³ of drainage gravel.

Geotextile fabric: Area = Wall length x (Wall height + 0.5m for base and top overlap). For a 10m x 1.0m wall: 10 x 1.5 = 15m².

Weep holes: One weep hole per 1.8–2.0m of wall length. For a 10m wall: 5–6 weep holes.

Browse our drainage kits for pre-packaged drainage solutions.

Quick Reference — 10m x 1.0m Wall

Get a Free Materials Quote

Rather have us do the maths? Book a free quote with your wall dimensions and we'll calculate exact quantities, recommend the right products, and provide a full delivered price — at no cost. You can also use our Retaining Wall Cost Calculator for a quick budget estimate.