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Concrete Calculator
Volume and bags for slabs, footings and posts.
What it does. Concrete volume is length times width times depth. The complications are that depth is usually the dimension people guess at, dry bag yield is much lower than bag weight suggests, and there is no practical way to add concrete to a pour that has started to set.
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How to use Concrete Calculator
- Choose a slab, footing or post hole.
- Enter dimensions in metric or imperial.
- Read volume, bag count, and the mix quantities.
Dry bags yield far less than they weigh
A 25kg bag of ready-mix produces roughly 0.012 cubic metres of wet concrete — around 12 litres. A cubic metre therefore needs something like 80 to 90 bags.
People consistently underestimate this, because the bag is heavy and the yield is not intuitive.
Above about half a cubic metre, mixing by bag stops making sense on both cost and effort. That is roughly the point where ready-mix delivery becomes cheaper as well as easier.
Mixing your own
| Use | Mix (cement : sand : aggregate) | Note |
|---|---|---|
| General purpose slab | 1 : 2 : 4 | Paths, bases, general |
| Foundations, footings | 1 : 2.5 : 3.5 | Structural |
| Heavy duty, driveways | 1 : 1.5 : 3 | Higher strength |
| Post setting | 1 : 2 : 4 or postcrete | Postcrete is faster |
| Mortar (not concrete) | 1 : 4 cement : sand | Bricklaying |
Ratios are by volume, not weight, and water is deliberately absent — it is added to workability rather than to a ratio. More water makes concrete easier to place and measurably weaker, which is the most common way a home-mixed pour ends up under strength.
Order more than the calculation
Add roughly 5 to 10%. Sub-base is never perfectly level, formwork bows slightly under the weight, and some is always lost in handling.
Running short mid-pour is a genuine problem: a cold joint between the first pour and a later top-up is a structural weakness and a visible line.
For post holes the usual guide is a hole three times the post width and a depth of about a third of the post height above ground, which this tool uses as its default.
Frequently asked questions
How many bags of concrete are in a cubic metre?
Roughly 80 to 90 bags of 25kg ready-mix, since each yields only about 12 litres. Above half a cubic metre, ready-mix delivery is usually cheaper as well as easier.
What mix ratio should I use?
1:2:4 cement, sand and aggregate for general slabs and paths; 1:2.5:3.5 for footings; 1:1.5:3 for driveways. Ratios are by volume, not weight.
How much extra concrete should I order?
5 to 10%. Sub-base is never perfectly level and formwork bows under the load. Running short creates a cold joint, which is both a weakness and a visible line.
How deep should a post hole be?
A common guide is a hole three times the post width, at a depth of about a third of the post height above ground — so a 2m fence post needs roughly 600mm in the ground.
Does adding water weaken concrete?
Yes, measurably. Extra water makes it easier to place and lowers the final strength, which is the most common way a home-mixed pour ends up weaker than intended.