Concrete calculator: how much concrete do I need?

As a rule of thumb, you need roughly 90 to 110 bags of 20kg premix concrete per cubic metre, and once you pass about half a cubic metre, ready-mix delivered by truck usually works out cheaper and easier.

Use the calculator below to work out the volume and bag count for your project.

Length (m) Width (m) Depth (mm)

How the numbers work

Slab Lab crew preparing and levelling the ground before a driveway pour

Bag yield

A 20kg bag of premix concrete yields roughly 0.009 to 0.01 cubic metres, which is why the rule of thumb is about 90 to 110 bags per cubic metre.

Waste allowance

Always add 5 to 10% extra to your calculated volume to cover spillage, uneven ground and over-excavation. Our calculator builds this in automatically.

When to switch to ready-mix

Once you're over roughly half a cubic metre, ordering ready-mix delivered by truck usually works out cheaper and far less labour than mixing dozens of bags by hand.

Standard depths

Paths and footings are typically 75 to 100mm deep. Driveways and structural slabs are usually 100mm or more with reinforcing, depending on the load.

These are estimates

This calculator gives you a planning figure. For anything structural, or a job of any real size, get a free on-site quote for an exact volume and mix specification.

Need an exact figure, not just an estimate?

Book a free, no-obligation on-site quote and we'll confirm the exact volume, mix and reinforcing your project needs, no guesswork required.

FREE QUOTE

FAQ

How much concrete do I need for a slab?

Multiply length by width by depth (in metres) to get the volume in cubic metres, then add 10% for waste. As a rule of thumb, budget roughly 90 to 110 bags of 20kg premix per cubic metre if you're using bagged concrete.

How many 20kg bags of concrete make a cubic metre?

Roughly 90 to 110 bags of 20kg premix concrete make one cubic metre, since each bag yields about 0.009 to 0.01 cubic metres once mixed.

When should I use ready-mix instead of bagged concrete?

Once your project needs more than about half a cubic metre, ready-mix delivered by truck is usually cheaper and much less labour than mixing dozens of bags by hand.

How deep should a concrete path or slab be?

Paths and footings are typically 75 to 100mm deep. Driveways and structural slabs are usually 100mm or thicker with steel reinforcing, depending on the load they need to carry.

Is this calculator accurate enough to order concrete?

It's a solid planning estimate, but for anything structural or larger than a small path, get a free on-site quote. We'll confirm the exact volume, mix and reinforcing your project needs.

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