Concrete & construction
Concrete Foundation Calculator
Works foundation concrete, where the design is an engineering question.
Result
A slab-on-grade foundation is thicker at its edges and under bearing walls, and this calculates the field of the slab only. Add the perimeter beam and any interior thickenings with the footing calculator, and take the sizes from the structural drawings rather than from a rule of thumb.
Get a Free Business AuditHow the calculation works
Footing volume plus wall or stem volume, each as a cross-section times a length, with reinforcement quantified separately. Foundation dimensions and reinforcement follow the structural requirement for the loads and the soil, which comes from the code tables or an engineer rather than from a quantity calculation.
What moves the number
- Footing and wall dimensions from the structural requirement.
- Perimeter length and any internal bearing lines.
- Reinforcement schedule.
- Waterproofing and drainage provision.
Where this one goes wrong
A foundation is the one element where a shortfall cannot be remedied afterwards without removing what sits on it, which is why the dimensions and reinforcement come from the requirement and the quantity calculation simply follows. Get the design right first. Drainage and waterproofing against the foundation also have to be installed as it is built, since neither can be added later without excavating back to it.
Before you quote
- Take all dimensions and reinforcement from the structural design.
- Install drainage and waterproofing as the work proceeds.
- Have the work inspected at the required stages.
Where the rules come from
Questions
About this calculator
Can I design a foundation from a calculator?
No. Dimensions and reinforcement follow the structural requirement for the loads and the soil, which comes from the code tables or an engineer. A quantity calculation follows the design rather than producing it.
Why does foundation work have to be right first time?
Because a shortfall cannot be remedied without removing what sits on it. It is the one element where there is no later opportunity to correct, which is why the inspections happen at specific stages as it is built.
When is waterproofing installed?
As the foundation is built, along with the perimeter drainage, since neither can be added afterwards without excavating back to it. Deferring either is a decision to excavate later at many times the cost.
The formula
cubic yards = area x thickness/12 / 27, plus waste
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How to use this calculator
- Length (ft) (ft). Enter your own figure; the page starts at 40 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 28 ft.
- Thickness (in) (in). Enter your own figure; the page starts at 4 in.
- Waste allowance (%). Enter your own figure; the page starts at 8%.
- Concrete per cubic yard ($). Enter your own figure; the page starts at $165.
- Labour per sq ft ($). Enter your own figure; the page starts at $6.
The result updates as you type, so you can try a figure, see what it does to cubic yards to order, 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) | 40 ft |
| Width (ft) | 28 ft |
| Thickness (in) | 4 in |
| Waste allowance | 8% |
| Concrete per cubic yard | $165 |
| Labour per sq ft | $6 |
What comes out
| Result | Value |
|---|---|
| Cubic yards to order main result | 15.0 |
| Foundation area | 1,120.0 |
| Concrete cost | $2,475 |
| Estimated total | $9,195 |
| Cost per square foot | $8.21 |
That is 15.0 cubic yards over 1,120.0 square feet, about $9,195 all in, or $8.21 a square foot.
How sensitive the answer is
Holding everything else steady and changing only concrete per cubic yard, this is what happens to cubic yards to order. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Concrete per cubic yard ($) | Cubic yards to order |
|---|---|
| $83 | 15.0 |
| $124 | 15.0 |
| $165 example above | 15.0 |
| $248 | 15.0 |
| $330 | 15.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Length (ft) (ft)
- Measured in ft, starting at 40 ft.
- Width (ft) (ft)
- Measured in ft, starting at 28 ft.
- Thickness (in) (in)
- Measured in in, between 0 and 120, starting at 4 in.
- Waste allowance (%)
- A percentage, between 0 and 40, starting at 8%.
- Concrete per cubic yard ($)
- A money amount, starting at $165.
- Labour per sq ft ($)
- A money amount, starting at $6.
What the results tell you
| Result | What it is |
|---|---|
| Cubic yards to order | The headline result. A decimal figure. |
| Foundation area | A decimal figure. |
| Concrete cost | A money figure. |
| Estimated total | A money figure. |
| Cost per square foot | A money figure. |
Order slightly more than this
Volume calculations assume a level, properly prepared base and exact dimensions. Sub-base variation, over-excavation and spillage all consume concrete, which is why most contractors add a margin. Running short mid-pour is far more expensive than a small overage.
Keep going
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