Contractor & construction
Concrete Slab Calculator
Works a slab, where thickness and reinforcement follow the load rather than the area.
Result
Excludes excavation, base gravel, forming, reinforcement and vapour barrier unless you fold them into the labour figure. A proper slab also needs a compacted base, which the gravel calculator sizes.
Get a Free Business AuditHow the calculation works
Slab volume from the area and thickness, plus the subbase material, plus reinforcement and any vapour retarder. Thickness and reinforcement are determined by what the slab carries and by the adopted code rather than by the slab's size, which makes those inputs rather than outputs.
What moves the number
- Intended load, which sets thickness and reinforcement.
- Subbase depth and compaction.
- Control joint spacing.
- Vapour retarder where the slab is interior or conditioned.
Where this one goes wrong
Control joints are what determine whether a slab cracks where you want it to or where it chooses, and the spacing relates to the thickness, so a slab poured without them will crack regardless of how good the concrete was. Plan them. The subbase matters as much as the concrete, since a slab on a poorly compacted base will crack and settle whatever its thickness, and that is the part nobody sees afterwards.
Before you quote
- Take thickness and reinforcement from the load and the adopted code.
- Compact the subbase properly before pouring.
- Plan control joint spacing relative to thickness.
Where the rules come from
Questions
About this calculator
How thick should a slab be?
It follows the load it carries and the adopted code rather than the slab's area. A patio, a driveway and a slab carrying vehicles or equipment have different requirements, and the local authority applies the adopted text.
Why do slabs crack?
Concrete shrinks as it cures, so it will crack; control joints determine whether it cracks where you chose. A slab poured without them cracks where it chooses, and the spacing relates to the thickness.
Does the subbase matter?
As much as the concrete. A slab on a poorly compacted base cracks and settles whatever its thickness, and it is the part nobody can see afterwards, which is why it is where corners get cut and where failures originate.
The formula
cubic yards = length × width × (depth ÷ 12) ÷ 27
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How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 24 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 24 ft.
- Slab thickness (inches) (in). Enter your own figure; the page starts at 5 in.
- Waste / compaction allowance (%). Enter your own figure; the page starts at 10%.
- Cost per cubic yard delivered ($). Enter your own figure; the page starts at $175.
- Labour and finishing per sq ft ($). Enter your own figure; the page starts at $4.5.
The result updates as you type, so you can try a figure, see what it does to cubic yards needed, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Length (ft) | 24 ft |
| Width (ft) | 24 ft |
| Slab thickness (inches) | 5 in |
| Waste / compaction allowance | 10% |
| Cost per cubic yard delivered | $175 |
| Labour and finishing per sq ft | $4.5 |
What comes out
| Result | Value |
|---|---|
| Cubic yards needed main result | 9.8 |
| Cubic feet | 264.0 |
| Approximate tons | 19.8 |
| Estimated cost | $1,711 |
| Slab area | 576.0 |
| Total installed cost | $4,303 |
A 24 × 24 slab at 5" takes 9.8 cubic yards. Concrete costs $1,711 and the installed total is about $4,303.
How sensitive the answer is
Holding everything else steady and changing only cost per cubic yard delivered, this is what happens to cubic yards needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Cost per cubic yard delivered ($) | Cubic yards needed |
|---|---|
| $88 | 9.8 |
| $131 | 9.8 |
| $175 example above | 9.8 |
| $263 | 9.8 |
| $350 | 9.8 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 24 ft.
- Width (ft) (ft)
- Measured in ft, starting at 24 ft.
- Slab thickness (inches) (in)
- Measured in in, starting at 5 in.
- Waste / compaction allowance (%)
- A percentage, between 0 and 60, starting at 10%.
- Cost per cubic yard delivered ($)
- A money amount, starting at $175.
- Labour and finishing per sq ft ($)
- A money amount, starting at $4.5.
What the results tell you
| Result | What it is |
|---|---|
| Cubic yards needed | The headline result. A decimal figure. |
| Cubic feet | A decimal figure. |
| Approximate tons | A decimal figure. |
| Estimated cost | A money figure. |
| Slab area | A decimal figure. |
| Total installed cost | A money figure. |
Overhead is where these go wrong
Pricing and margin figures are only as good as the cost you put in. If your cost covers parts and wages but not the van, the insurance, the phone and the unbilled hours, the markup is paying for those before it becomes profit, and the margin you think you are making is not the one you are making.
Keep going
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