Concrete Calculator Australia

How to calculate how much concrete you need

Concrete volume is normally calculated in cubic metres (m³). For a rectangular slab: Length × Width × Depth = Cubic metres. Convert the depth to metres first: 100 mm = 0.1 m. A 6 m × 4 m slab at 100 mm thick is 6 × 4 × 0.1 = 2.4 m³.

This concrete volume calculator also handles strip footings, round slabs and post holes. For round shapes, use π × radius² × depth, then multiply by the quantity. All dimensions are converted to metres before calculating.

Concrete slab calculation examples

These are illustrative volume calculations only. They do not recommend a thickness for any structure.

Small slab

3 m × 3 m × 100 mm: 3 × 3 × 0.1 = 0.90 m³.

6 m × 4 m slab

6 × 4 × 0.1 = 2.40 m³.

10 m × 5 m slab at 120 mm thick

10 × 5 × 0.12 = 6.00 m³.

Concrete footing calculator

For a rectangular footing or trench, multiply length by width by depth and by the number of identical runs. A fictional 10 m run, 400 mm wide and 300 mm deep, has a base volume of 10 × 0.4 × 0.3 = 1.20 m³. Convert width and depth from millimetres first. These dimensions are examples, not footing-design advice.

Post hole concrete calculator

Round holes use π × radius² × depth × number of holes. Ten fictional holes with a 300 mm diameter and 600 mm depth have a combined base volume of approximately 0.4241 m³. Round columns use the same cylinder geometry with height in metres. No post displacement is deducted; check the actual fill volume for the project.

How much extra concrete should I order?

The calculator starts with a visible 10% waste allowance. Choose 0%, 5%, 10%, 15% or a custom value for your job. Base volumes are added at full precision and waste is applied once to that total. For a 2.40 m³ slab with 10% waste, the required volume is 2.64 m³. The separate planning order rounds up to your selected 0.1 m³ or 0.01 m³ increment. This is not a supplier ordering rule: confirm quantities, minimum loads and delivery requirements with the supplier.

Steps, walls and kerb profiles

Walls are rectangular volumes. Steps assume equal treads and risers with solid fill down to a level base; landings and sloping undersides are excluded. Kerb and gutter assume two adjacent rectangular areas, without counting an overlap twice. The calculator explains these assumptions beside the fields. Curved, tapered or engineered profiles need their actual project geometry.

Why might I need more concrete than the calculator shows?

The result is a mathematical volume estimate. Uneven excavation, over-excavation, ground variation, formwork dimensions, footing variation, spills, measurement errors and other site conditions can change the actual amount required.

An optional allowance can help you explore the difference, but 5% or 10% is not correct for every job. Confirm dimensions and quantities with the people responsible for your project and your concrete supplier.

What concrete thickness should I use?

The required thickness depends on the type of project, loading, ground conditions, reinforcement, design and any applicable engineering or construction requirements.

Jirvo’s calculator calculates volume only. It does not determine the correct structural thickness for a project. For structural work, follow the relevant project plans, engineering or professional requirements.

How to convert concrete thickness from millimetres to metres

Divide millimetres by 1,000. The calculator handles this conversion automatically.

• 50 mm = 0.05 m
• 75 mm = 0.075 m
• 100 mm = 0.10 m
• 120 mm = 0.12 m
• 150 mm = 0.15 m
• 200 mm = 0.20 m

Planning the rest of your quote

Once you have a concrete quantity, use a free Australian tradie quote template to organise the job scope. Check your labour assumptions with the Australian tradie hourly-rate guide, allow for commonly forgotten job costs, and understand how to mark up materials on a trade quote.