When you’re starting a DIY project, whether it’s a new driveway, a garden pathway, or the foundation for a backyard shed, one question always comes up: “How much concrete do I actually need?” Getting this calculation right can save time, money, and frustration.
Ordering too little concrete can bring your project to a sudden halt, while ordering too much can lead to unnecessary costs and wasted materials. Most concrete suppliers recommend measuring carefully and planning ahead to avoid surprises. Fortunately, estimating the correct amount isn’t complicated once you understand a few basic concepts.
Understanding How Concrete Is Measured
In Australia, concrete is typically measured in cubic metres (m³). One cubic metre represents a volume that’s 1 metre long, 1 metre wide, and 1 metre deep. Most suppliers sell ready-mix concrete by the cubic metre, so knowing your exact volume requirements ensures you order the correct quantity.
Step 1: Measure Your Project Area
The first step is to measure the length, width, and depth (thickness) of the area you want to concrete. These measurements will form the basis of your concrete volume calculation.
For example:
- Driveway: Measure the full length and width of the area, and decide on the thickness (usually 100–125 mm for cars, 150 mm for heavier vehicles).
- Patio or path: Typically, a thickness of 75–100 mm is sufficient.
- Concrete slab for a shed or floor: Commonly 100 mm thick.
Always measure in metres. If your tape measure shows centimetres or millimetres, convert these to metres for easier calculation (e.g. 100 mm = 0.1 m).
Step 2: Calculate the Concrete Volume
To find the volume, multiply the three measurements together:
Volume = Length × Width × Depth
For example:
If you’re building a 4 m × 3 m patio with a 0.1 m thickness:
4 × 3 × 0.1 = 1.2 cubic metres of concrete.
If your area isn’t perfectly rectangular — such as a circular slab or curved path — you can use shape-specific formulas or a concrete calculator to make the process easier.
- Circular slab: π × radius² × depth
(e.g. a 3 m diameter slab, 0.1 m thick → 3.14 × 1.5² × 0.1 = 0.71 m³)
Step 3: Allow for Wastage
It’s wise to allow for extra concrete to account for spillage, uneven ground, or variations in slab thickness. Most builders recommend adding 5–10% to your calculated total.
For example:
1.2 m³ × 1.1 (10% extra) = 1.32 cubic metres
Rounding up slightly is better than running short halfway through your pour — especially if the truck has already left the site!
Step 4: Consider the Type of Concrete Mix
Different concrete mixes have varying strength and workability, depending on your project. When ordering, you’ll need to specify:
- Strength grade (e.g. 20 MPa for light domestic work, 25–32 MPa for structural slabs)
- Aggregate size (larger stones for slabs, finer mixes for smooth surfaces)
- Slump (how wet or workable the mix is)
Your local concrete supplier can advise on the best mix for your application. It’s worth noting that high-strength mixes may require less thickness to achieve the same performance — but always confirm with an engineer or supplier before adjusting your plans.
Step 5: Use an Online Concrete Calculator
To simplify things, you can use an online concrete calculator. These tools help you input your project’s dimensions and instantly estimate the required volume in cubic metres. Many Australian suppliers, such as Boral, Holcim, and Concrete Taxi, offer free calculators on their websites.
They also often recommend how many 20 kg pre-mixed bags you’d need if you’re mixing by hand, perfect for smaller DIY jobs like garden edging or post holes.
Step 6: Plan for Delivery and Pouring
Once you know how much concrete you need, it’s time to plan logistics:
- Order from a local supplier: Ready-mix concrete is delivered in trucks that usually carry between 0.4 and 6 cubic metres.
- Check access: Make sure the truck can reach the pour site. If not, you may need a concrete pump.
- Prepare your site: Have formwork and reinforcement ready before the truck arrives. Concrete sets quickly!
The Bottom Line
Calculating how much concrete you need isn’t complicated, it’s simply a matter of measuring accurately, doing a quick volume calculation, and allowing a little extra for waste. Whether you’re laying a new driveway in Sydney, a garden path in Brisbane, or a shed slab in Perth, knowing your concrete requirements upfront will save you time, money, and stress.
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