Volume Formulas
Cylindrical: π × (Diameter / 2)² × Depth
M3 Calculator — Cubic Metre Calculator for Builders
Australian residential builds consume an average of 20–30 cubic metres of concrete per project (HIA, 2025), and that’s before gravel, topsoil, and mulch. Calculating cubic metres (m³) accurately is the foundation of every material order on site.
How to Calculate Cubic Metres (m³)
Rectangular volumes (slabs, footings, garden beds):
m³ = Length (m) × Width (m) × Depth (m)
Cylindrical volumes (piers, stumps, round tanks):
m³ = π × r² × Height (m) (where r = radius in metres)
Common Quantities — Cubic Metres per Job
| Application | Typical Depth | m³ per m² | 10m² Example |
|---|---|---|---|
| Concrete slab (standard) | 100mm | 0.10 | 1.0 m³ |
| Concrete slab (heavy duty) | 150mm | 0.15 | 1.5 m³ |
| Gravel base / sub-base | 75mm | 0.075 | 0.75 m³ |
| Topsoil (turf prep) | 100mm | 0.10 | 1.0 m³ |
| Mulch (garden beds) | 75mm | 0.075 | 0.75 m³ |
| Sand bedding (pavers) | 40mm | 0.04 | 0.40 m³ |
Waste Allowance Guide
| Material | Recommended Waste % | Why |
|---|---|---|
| Ready-mix concrete | 5–10% | Over-excavation, uneven formwork |
| Gravel / crushed rock | 10–15% | Compaction loss, spreading |
| Topsoil | 10% | Settlement after watering |
| Mulch | 5–10% | Wind loss, settling |
| Sand | 10% | Compaction, spillage |
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Frequently Asked Questions
How do you calculate m3 for concrete?
Multiply the length by the width by the depth, all in metres. For example, a 5m × 3m slab at 100mm (0.1m) depth = 5 × 3 × 0.1 = 1.5 m³. Always add 5–10% waste allowance for concrete.
How many 20kg bags of concrete per m3?
A 20kg bag of pre-mixed concrete yields approximately 0.009 m³. That means you need roughly 108 bags to fill one cubic metre. For anything over 0.5 m³, ready-mix delivery is far more economical.
What is the difference between m3 and cubic metres?
There is no difference — m³ is simply the abbreviation for cubic metres. Both refer to the same unit of volume used to measure concrete, soil, gravel, and other bulk materials in Australia.
How many m3 of concrete for a house slab?
An average Australian house slab (150–200 m²) at 100mm depth requires 15–20 m³ of concrete, plus edge beams which add another 3–8 m³ depending on the design. Total is typically 20–30 m³ (ABS Building Activity data, 2024).