Free tool
Concrete calculator
Length, width and depth in, cubic metres and bag count out.
Concrete is sold by the cubic metre and measured in millimetres, which is where most of the arithmetic errors come from. Put the dimensions in above and it converts, adds an allowance, and tells you whether the job is a bag job or a truck job.
The sum
Volume is length times width times depth, all in metres. A 6m by 4m slab at 100mm is 6 x 4 x 0.1, which is 2.4 cubic metres. The trap is the depth: 100mm is 0.1m, not 100.
Why the allowance matters
Ground is never as flat as the plan. Formwork bows, sub base dips, and some of the load stays in the barrow and the chute. Ten per cent is the usual allowance on a domestic slab and it is the difference between finishing the pour and standing in a half finished slab ringing a supplier who has gone home.
Running out is far worse than having a little over. A cold joint through the middle of a slab because the second load arrived an hour later is a permanent weakness, not a cosmetic one.
Bags or a truck
A 20kg bag of premix makes about 9 litres of wet concrete, so it takes roughly 111 bags to make a cubic metre. That maths turns most people off bags quickly.
Under about a third of a cubic metre, bags are sensible: post holes, a small pad, a set of steps. Past that, the bag count, the mixing time and the cost all point at a truck, and most suppliers have a minimum load with a short load fee below it. Ring them with the volume rather than the dimensions and they will tell you.
Depths that are normal
| Job | Typical depth |
|---|---|
| Garden path or small pad | 75 to 100mm |
| Domestic driveway, cars only | 100mm |
| Driveway with truck or caravan access | 125 to 150mm |
| Shed or garage slab | 100mm on a prepared base |
| Fence post hole | 600mm deep, 300mm wide, as a starting point |
Anything structural, anything on reactive clay, and anything a building surveyor will look at is a job for an engineer rather than a table on a website. Both states require certain concrete work to be done or supervised by a licensed practitioner, and the licence check opens the right register.
Getting it delivered on the border
Batching plants serve a radius and concrete has a working life measured in a couple of hours, so how far you are from the plant matters more than it does for other materials. Suppliers on the Albury Wodonga corridor deliver both sides of the river, but the travel time comes out of the working time, and a late Friday pour in summer is a different proposition to a Tuesday morning one.
Common questions
How much concrete do I need for a 6m by 4m slab?
At 100mm deep, 2.4 cubic metres before any allowance, and about 2.64 with a 10% allowance. Multiply length by width by depth in metres, so 6 x 4 x 0.1. The calculator above does it for any dimensions.
How many 20kg bags of concrete make a cubic metre?
About 111. Each 20kg bag yields roughly 9 litres of wet concrete. That is why bagged premix only makes sense under about a third of a cubic metre, and why anything larger is a truck job.
How much extra concrete should I order?
Ten per cent is the usual allowance on a domestic pour, for uneven ground, formwork movement and what stays in the barrow. Running out mid pour creates a cold joint, which is a permanent weakness rather than a cosmetic one, so over ordering slightly is the cheap option.
How deep should a concrete driveway be?
Around 100mm for cars, and 125 to 150mm where a truck, caravan or boat trailer will run over it. Depth is only half the story: what is under the slab matters as much, and reactive clay soils change the answer entirely.
Do I need a licensed concreter?
It depends on the value and nature of the work and which state it is in. Both New South Wales and Victoria set a threshold above which residential building work needs a licensed practitioner, and structural work is a different question again. The licence check tool opens the relevant register.