Whether you're pouring a shed slab, setting fence posts, or laying a path, the two questions are always the same: how much concrete, and what will it cost? This guide works both out in Australian metric terms — cubic metres, 20kg bags, and ready-mix loads.
Concrete in Australia is measured, sold, and delivered in cubic metres (m³). Every estimate starts by turning your slab or footing into a volume, and the formula is simply length × width × depth, with every measurement in metres.
The step that trips people up is depth. A slab thickness of 100mm is 0.1 metres, not 100. Convert millimetres to metres before multiplying, or the answer comes out a thousand times too large.
Volume (m³) = Length (m) × Width (m) × Depth (m)
A typical single-car shed slab of 6m × 3m at 100mm thick works out to 6 × 3 × 0.1 = 1.8 m³. A garden path 10m long, 1m wide and 75mm thick is 10 × 1 × 0.075 = 0.75 m³. Our concrete calculator does this conversion for you and handles slabs, footings, columns and post holes in one place.
For an L-shaped or irregular pour, split the area into rectangles, calculate each separately, and add them together. It is more reliable than trying to average an odd shape in your head.
Australia gives you two ways to buy concrete, and the right choice is almost entirely a question of volume.
Pre-mixed bags — typically 20kg from Bunnings or a hardware store — suit small jobs. A 20kg bag yields only around 0.01 m³ of set concrete, which means our 1.8 m³ shed slab would need roughly 180 bags. That is around two tonnes of bags to lift, mix and place by hand, usually before the first batch starts going off.
Ready-mix is delivered by truck, mixed to a consistent specified strength, and placed in one continuous pour. It is the standard choice for slabs and larger footings. Suppliers generally quote per cubic metre, often with a minimum load charge and a part-load fee for anything under a full truck.
As a rough guide: post holes and small repairs suit bags; paths and small pads sit in the grey zone; anything shed-sized or larger is ready-mix territory.
Fence posts, clothes lines, and pergola footings are the most common small concrete job, and they are a case where bags genuinely win.
A post hole is a cylinder, so its volume is π × radius² × depth. A common hole of 300mm diameter (150mm radius) dug 600mm deep holds about 0.042 m³ — a little over four 20kg bags per hole, before allowing for the volume the post itself displaces.
Multiply by the number of holes, then round up. For a fence with ten posts you are looking at roughly 40 bags, which is manageable by hand over a day. Our concrete calculator has a post-hole mode that takes diameter and depth directly.
Your calculated volume is the concrete that ends up in the form if everything goes perfectly. It never does, so you order more.
A wastage allowance of around 5% to 10% is standard. Applying 10% to our 1.8 m³ slab means ordering close to 2.0 m³. Running out mid-pour is far worse than a small over-order: a cold joint between a first pour and a second delivery is a structural weakness, so it is better to have slightly too much than to stop.
Cost is the question most people are really asking, and it has two components: the concrete itself and everything around it.
Ready-mix concrete is priced per cubic metre, and the rate varies with the specified strength (commonly 20, 25 or 32 MPa), your location, and how far the truck travels. Prices move over time and between suppliers, so the only reliable figure is a current quote from a local batching plant for your exact mix.
Watch for the charges that don't appear in the per-metre rate: minimum load fees, part-load surcharges for less than a full truck, waiting time if the site isn't ready, and after-hours or Saturday delivery premiums. Ask the supplier to itemise these before you book.
Because prices change, we don't publish a rate here — use our concrete calculator to lock in your volume, then get two or three local quotes against that figure.
How do I calculate how much concrete I need?
Multiply length by width by depth, with every measurement in metres, to get the volume in cubic metres. For example, a 6m by 3m slab at 100mm (0.1m) thick is 6 × 3 × 0.1 = 1.8 cubic metres. Then add 5 to 10 per cent for wastage before ordering.
How many 20kg bags of concrete are in a cubic metre?
A 20kg bag of pre-mixed concrete yields roughly 0.01 cubic metres when set, so approximately 100 bags make up one cubic metre. This is why bags only suit small jobs — a standard shed slab would need well over 150 bags.
When should I use ready-mix instead of bags?
Bags suit small jobs such as post holes and repairs. Once you pass roughly half a cubic metre, hand-mixing becomes slow and inconsistent, and ready-mix delivered by truck is generally the better choice for slabs and larger footings.
How much concrete for a fence post hole?
A post hole 300mm in diameter and 600mm deep holds about 0.042 cubic metres, or a little over four 20kg bags, before accounting for the space the post itself takes up. Multiply by the number of holes and round up.
Why should I order extra concrete?
Uneven ground, spillage, and material that can't be scraped from the mixer or truck all mean the concrete placed is a little more than calculated. A 5 to 10 per cent allowance prevents running out mid-pour, which would create a weak cold joint.
How much does concrete cost in Australia?
Ready-mix is priced per cubic metre and varies by strength, location and delivery distance, so a current local quote is the only reliable figure. Remember the concrete is often not the largest cost — formwork, reinforcement, base preparation, pumping and labour frequently add up to more.