How to Calculate Concrete for Any Project in Australia
Whether you're pouring a slab, setting fence posts, building footings, or casting columns, getting the right amount of concrete is the first step to a successful project. Each project type uses a different volume formula, but the principle is the same: calculate the shape's volume in cubic metres, add wastage, and convert to bags or ready-mix.
Concrete for Different Project Types
Australian DIYers and tradies work with concrete across a wide range of projects. Rectangular slabs are the most common - from shed bases to driveways - and use a simple length × width × depth formula (see our dedicated slab calculator for detailed guidance). Post holes for fences, decks, and pergolas use a cylindrical formula (π × radius² × depth). Strip footings for retaining walls and house extensions calculate as length × width × depth of the trench. Columns and piers can be round or square, each with their own volume calculation. Our suite of calculators handles all of these automatically.
Understanding Concrete Volume (m³)
All concrete in Australia is measured in cubic metres (m³). One cubic metre equals 1,000 litres of mixed concrete. To calculate volume, measure all dimensions in metres (convert millimetres by dividing by 1,000). For rectangular shapes, multiply length × width × depth. For cylindrical shapes like post holes, use π × radius² × depth. The shape of your project determines which formula and which calculator you need.
Why Wastage Matters
Adding 10% wastage to any concrete calculation is standard practice. Uneven ground requires more concrete in low spots. Spillage happens during mixing, transporting, and pouring. Some concrete sticks to equipment and wheelbarrows. Minor measurement variations also consume material. For complex shapes, sloping sites, or jobs with lots of post holes, increase wastage to 15%. Running short mid-pour is far worse than having a small amount left over.
Bags vs Ready-Mix: Which to Choose
The decision depends on total volume and project type. For small jobs under 0.3m³ - like a few post holes or a small garden path - 20kg bags from Bunnings are practical and convenient. For medium projects between 0.3 and 0.8m³, either option works, but ready-mix saves significant labour. For anything over 0.8m³, ready-mix delivery is strongly recommended. Our bags vs ready-mix comparison tool gives you a detailed cost breakdown for any volume.
Choosing the Right Concrete Strength
Concrete strength in Australia is rated in megapascals (MPa). For non-structural applications like garden paths and post holes, 20 MPa is sufficient. Standard residential projects - patios, shed bases, and fence footings - typically use 25 MPa. Driveways, garage floors, and structural footings require 32 MPa to handle vehicle loads and structural demands. Most bagged concrete from hardware stores is 20-25 MPa, while ready-mix suppliers can provide any strength grade to specification.
Common Australian Concreting Projects
The most popular residential concrete projects include shed bases (typically 3×3m to 6×4m), driveways (3m wide, varying lengths), patios and entertainment areas, fence post holes (300-450mm diameter), deck and pergola footings, garden paths, and garage floors. Each project type has specific requirements for thickness, reinforcement, and concrete strength. Use our dedicated calculators for accurate estimates tailored to your specific project.
Pre-Calculated Project Guides
Jump straight to calculations for popular Australian projects: