Concrete Calculator Australia
Planning a backyard slab or setting fence posts? Quickly calculate your required cubic meters and standard 20kg bags.
Calculate Concrete for Slabs and Post Holes
G'day! Whether you are laying down a new concrete slab for a shed in the backyard or digging post holes for a new fence, getting your concrete volume right is crucial in Australia. Order too little from the ready mix truck, and you are hit with short load fees. Buy too many bags from the hardware store, and you are wasting your hard earned cash.
Using our Australian concrete calculator is simple:
- Select what you are building: a slab, post holes or columns, or stairs.
- Enter your measurements. The tool is set to metric, so use meters for length and width, and centimeters for the depth or thickness.
- Hit calculate to get your exact cubic meters and see how many standard 20kg bags you need.
How Do You Calculate Cubic Meters of Concrete?
If you want to do the math yourself without the calculator, calculating concrete volume in cubic meters is quite straightforward. You just need to ensure all your measurements are in meters before multiplying.
The Formula: Length in meters multiplied by Width in meters multiplied by Depth in meters equals Total Cubic Meters.
Example for an Aussie driveway: Let us assume you are pouring a driveway section that is 5 meters long, 3 meters wide, and 100 millimeters thick. First, convert your thickness to meters by dividing by 1000, which gives you 0.1 meters. Now multiply: 5 times 3 times 0.1 equals 1.5 cubic meters of concrete.
20kg Concrete Bag Calculator and Yields
If you are doing a small job, you will likely pick up premixed bags of concrete from places like Bunnings or Mitre 10. In Australia, the most common bag size is 20kg. Here is what you can expect a single bag to yield once mixed with water:
| Australian Bag Size | Approximate Yield in Cubic Meters | Bags needed per cubic meter |
|---|---|---|
| Standard 20kg Bag | 0.0091 cubic meters | approx 110 bags |
| 30kg Bag | 0.0138 cubic meters | approx 73 bags |
Frequently Asked Questions
How many 20kg bags of concrete are in a cubic meter?
You will need approximately 108 to 110 standard 20kg bags of premixed concrete to make up one full cubic meter. For anything over 0.5 cubic meters, it is usually cheaper and much easier on your back to order a ready mix delivery rather than mixing bags.
How to calculate concrete for post holes?
To calculate concrete for post holes, treat the hole as a cylinder. Use our calculator above and select the Post Holes option. Generally speaking, a standard 600mm deep hole with a 200mm diameter will take about two to three 20kg bags of rapid set concrete.
What is the cost of a concrete slab?
While we do not provide a cost calculator, as prices fluctuate constantly across Australian states, you can expect to pay anywhere from $250 to $350 per cubic meter for standard ready mix concrete delivery, excluding the labour for pouring and finishing.
Should I add extra to my calculation?
Yes. Always add an extra 10 percent to your total volume. This covers slight variations in the depth of your excavation, spillage, and waste during the pour.