Work out how much cement, sand and aggregate — or how many bags of ready-mix concrete — you need for a slab, footing, path or post hole, in cubic metres and 20 kg bags.
| Item | Value |
|---|
Crushed rock and gravel are essential materials in Australian construction and landscaping. They are sold by weight (tonne) or volume (m³) depending on the supplier.
| Application | Material | Depth |
|---|---|---|
| House slab | Concrete 1:2:4 | 100 mm |
| Shed / garage floor | Concrete | 100–150 mm |
| Garden path | Concrete | 75–100 mm |
| Fence post hole | Rapid-set / concrete | Depth of hole |
| Footing / strip footing | Concrete | Per engineer |
| Gravel Type | Bulk Density | Common Use |
|---|---|---|
| Pea gravel / river pebbles | ~1.5-1.6 t/m³ | Garden beds, decorative |
| Crushed rock (20mm) | ~1.8-1.9 t/m³ | Driveways, drainage |
| Blue metal (20mm) | ~1.9-2.0 t/m³ | Concrete aggregate, driveways |
| Decomposed granite | ~1.6-1.8 t/m³ | Paths, driveways |
| Road base (compacted) | ~2.0-2.2 t/m³ | Sub-base for driveways |
| Application | Recommended Depth |
|---|---|
| Decorative garden bed | 40-50 mm |
| Landscaping path | 50-75 mm |
| Residential driveway | 75-100 mm over compacted base |
| Drainage layer | 100-150 mm |
How many bags of cement do I need per cubic metre?
As a rough guide, one cubic metre of standard concrete needs about 108 × 20 kg bags of pre-mixed concrete, or roughly 6–7 bags of cement plus sand and aggregate if mixing your own at a 1:2:4 ratio. This cement calculator gives you the volume in cubic metres so you can convert to bags for your chosen product.
How do I calculate concrete for a slab?
Multiply length × width × thickness (all in metres) to get the volume in cubic metres, then add about 10% for wastage and spillage. For example, a 4 m × 3 m slab at 100 mm is 4 × 3 × 0.1 = 1.2 m³. Enter your dimensions above and the calculator does it for you.
Should I use bagged concrete or order ready-mix?
For small jobs under about 1 cubic metre — post holes, small paths, repairs — bagged concrete is convenient. For larger pours like a shed slab, ordering ready-mix delivered by truck is usually cheaper and gives a more consistent result. The calculator shows your total volume so you can compare.