Concrete & construction
Footing Calculator
Works footing concrete, where the dimensions come from the code rather than the calculation.
Result
Footing size is a structural decision set by the load above, the soil bearing capacity and the frost depth where you are building. This works out the concrete for a footing that has already been specified. Over-excavation is the usual reason the real pour exceeds the calculated volume, which is why the waste allowance matters more on footings than on slabs.
Get a Free Business AuditHow the calculation works
Footing volume is the cross-section times the length for a strip footing, or the dimensions multiplied for a pad. The width, depth and reinforcement are determined by the load and the soil under the adopted code, which means they are inputs taken from the requirement rather than outputs of a quantity calculation.
What moves the number
- Required width and depth from the code and the load.
- Length of strip footing or number of pads.
- Frost depth, which sets the minimum bearing depth.
- Soil bearing capacity.
Where this one goes wrong
Footings have to bear below the local frost line, and a footing above it will heave with freezing ground and lift the structure regardless of how well it was poured, which makes frost depth a requirement rather than a preference. It varies by region and is set locally. Over-excavation is the other practical point, since a trench dug deeper or wider than specified consumes more concrete and nobody takes it back.
Before you quote
- Take width, depth and reinforcement from the adopted code.
- Bear below the local frost line.
- Keep the excavation to the specified dimensions.
Where the rules come from
Questions
About this calculator
How deep do footings need to be?
Below the local frost line, which varies by region and is set locally, and deep enough to bear on suitable soil. A footing above the frost line heaves with freezing ground and lifts the structure however well it was poured.
What determines footing width?
The load and the soil bearing capacity under the adopted code. It is a requirement taken from the code tables or an engineer rather than a figure chosen for convenience, and the local authority applies the adopted text.
Why does my footing use more concrete than calculated?
Over-excavation, usually. A trench dug deeper or wider than specified fills with concrete and nobody takes it back, which is why trenching to the dimensions rather than approximately is worth the care.
The formula
volume = (width/12) x (depth/12) x length / 27
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How to use this calculator
- Total footing length (ft) (ft). Enter your own figure; the page starts at 120 ft.
- Footing width (in) (in). Enter your own figure; the page starts at 16 in.
- Footing depth (in) (in). Enter your own figure; the page starts at 8 in.
- Waste allowance (%). Enter your own figure; the page starts at 10%.
- Price per cubic yard ($). Enter your own figure; the page starts at $165.
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 |
|---|---|
| Total footing length (ft) | 120 ft |
| Footing width (in) | 16 in |
| Footing depth (in) | 8 in |
| Waste allowance | 10% |
| Price per cubic yard | $165 |
What comes out
| Result | Value |
|---|---|
| Cubic yards to order main result | 4.5 |
| Net volume | 4.0 |
| Concrete per linear foot | 0.9 |
| Estimated cost | $742.50 |
A 16 by 8 inch footing running 120 feet takes 4.5 cubic yards, about $742.50.
How sensitive the answer is
Holding everything else steady and changing only price per cubic yard, this is what happens to cubic yards to order. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Price per cubic yard ($) | Cubic yards to order |
|---|---|
| $83 | 4.5 |
| $124 | 4.5 |
| $165 example above | 4.5 |
| $248 | 4.5 |
| $330 | 4.5 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Total footing length (ft) (ft)
- Measured in ft, starting at 120 ft.
- Footing width (in) (in)
- Measured in in, between 0 and 96, starting at 16 in.
- Footing depth (in) (in)
- Measured in in, between 0 and 96, starting at 8 in.
- Waste allowance (%)
- A percentage, between 0 and 40, starting at 10%.
- Price per cubic yard ($)
- A money amount, starting at $165.
What the results tell you
| Result | What it is |
|---|---|
| Cubic yards to order | The headline result. A decimal figure. |
| Net volume | A decimal figure. |
| Concrete per linear foot | A decimal figure. |
| Estimated cost | 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.
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