Work out how much concrete — or how many bags — you need for a slab, footing, path or post hole, in cubic metres/20kg bags (metric) or cubic yards/80lb bags (US).
Enter your pour dimensions below and pick your units — the calculator works out the concrete volume, the number of bags, and an estimated cost, whether you're working in m³ (AU) or cubic yards (US).
| 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 |
A shed slab 4 metres by 3 metres at 100 mm thick has a volume of 4 × 3 × 0.10 = 1.2 m³. Adding 10% for wastage and uneven ground gives about 1.32 m³. At roughly 108 bags of 20 kg pre-mix per cubic metre, that’s about 143 bags — a lot of mixing by hand, so for a slab this size ordering 1.3 m³ of ready-mix delivered would be cheaper and far easier.
One of the biggest cost decisions on any concreting job is whether to mix bags yourself or order ready-mix delivered by truck. The rule of thumb is volume. For small jobs under about one cubic metre — fence-post footings, a small path, repairs, or a garden edge — bagged concrete makes sense. It’s convenient, you mix only what you need, and there’s no minimum order. But mixing is hard physical work, and the cost per cubic metre is high once you’re buying dozens of bags.
For anything larger — a shed slab, driveway, or house footing — ready-mix is almost always cheaper per cubic metre and vastly less effort, and it gives a more consistent, stronger result because it’s batched to spec. The trade-off is that most suppliers have a minimum delivery (often around 1 m³) and charge extra for small loads, so very small jobs don’t justify it. This calculator gives you the exact volume so you can price both options: multiply the cubic metres by about 108 to get 20 kg bags, then compare that bag cost against a ready-mix quote for the same volume before deciding.
| 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 (1.5-2 in) |
| Landscaping path | 50-75 mm (2-3 in) |
| Residential driveway | 75-100 mm (3-4 in) over compacted base |
| Drainage layer | 100-150 mm (4-6 in) |
Concrete is measured and bagged differently depending on the market. Australia works in cubic metres (m³) and the standard bag size is 20 kg. The United States works in cubic yards, and the standard bag sizes are 60 lb and 80 lb. Use the Metric/US toggle above the calculator to switch — it converts your dimensions, volume, bag count and cost automatically.
| Measure | Australia / Metric | United States |
|---|---|---|
| Volume unit | Cubic metre (m³) | Cubic yard (cu yd) |
| Standard bag size | 20 kg | 60 lb / 80 lb |
| Bags per unit volume | ~108 × 20kg bags per m³ | ~45 × 80lb bags per cu yd |
| Conversion | 1 m³ = 1.308 cu yd | 1 cu yd = 0.765 m³ |
| Typical price | Priced per m³ delivered | Priced per cu yd delivered |
How many bags of cement do I need?
In metric, one cubic metre of standard concrete needs about 108 × 20 kg bags of pre-mix. In US units, one cubic yard needs about 45 × 80 lb bags (or roughly 60 × 60 lb bags). This calculator gives you the exact bag count in whichever unit system you choose.
How do I calculate concrete for a slab?
Multiply length × width × thickness to get the volume, 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 (about 1.57 cubic yards). Enter your dimensions above and the calculator does the rest.
Should I use bagged concrete or order ready-mix?
For small jobs — post holes, small paths, repairs — bagged concrete (20 kg or 60-80 lb bags) is convenient. For larger pours like a slab or driveway, ordering ready-mix delivered by truck is usually cheaper per unit volume and gives a more consistent result. The calculator shows your total volume so you can compare either way.
How much does concrete weigh?
Standard concrete weighs about 2.4 tonnes per cubic metre (roughly 4,000 lbs / 2 US tons per cubic yard) once cured. A modest 1.2 m³ (1.5 cu yd) slab weighs close to 3 tonnes, so the ground and any formwork must be able to support it.
How many yards of concrete do I need for a slab?
Switch the calculator to US units, enter your slab in feet and inches, and it converts directly to cubic yards. A typical 10×10 ft slab at 4 in thick needs about 1.2 cubic yards.
How do I work out concrete for a footing?
Multiply the footing length by its width and depth to get the volume. Strip footings are long and narrow, so measure carefully — and always follow the depth and width your engineer or local plans specify.
Can I mix concrete by hand for a slab?
For very small slabs it's possible, but it's hard work and hard to keep consistent. Above about one cubic metre (1.3 cu yd), ordering ready-mix delivered gives a stronger, more uniform result for less effort and often less money.