Concrete & construction
Concrete Volume Calculator
Works concrete volume for a shape, where irregular areas break into rectangles.
Result
Concrete is ordered by the cubic yard and delivered in fixed increments, so the figure you order is always rounded up. Most crews add 5% to 10% for spillage, uneven subgrade and over-excavation; running short mid-pour is far more expensive than a little waste. A 60 lb bag yields roughly 0.45 cubic feet, so bag counts rise very quickly with volume.
Get a Free Business AuditHow the calculation works
Volume is area times thickness, with the area calculated from the shape. An irregular area is handled by dividing it into rectangles and triangles, calculating each and summing, which is more reliable than attempting a single formula and is how a site measurement is usually taken anyway.
What moves the number
- Shape of the area and how it divides.
- Thickness, which may vary across the pour.
- Thickened edges or beams within the slab.
- Waste and subgrade allowance.
Where this one goes wrong
Thickened edges, perimeter beams and any haunching add volume that an area-times-thickness calculation misses entirely, and on a slab with a thickened perimeter that addition is substantial rather than marginal. Calculate them separately and add them. Where the thickness varies across a pour, using an average thickness is correct arithmetic only if the variation is genuinely linear.
Before you quote
- Divide irregular areas into rectangles and triangles.
- Add thickened edges and beams separately.
- Check whether an average thickness is genuinely representative.
Questions
About this calculator
How do I calculate an irregular area?
Divide it into rectangles and triangles, calculate each and sum. It is more reliable than attempting a single formula, and it matches how a site measurement is taken anyway, which makes checking the figure straightforward.
Do thickened edges matter?
Substantially. A perimeter beam or thickened edge adds volume that an area-times-thickness calculation misses entirely, and on a slab designed that way the addition is a meaningful share rather than a rounding allowance.
What if the thickness varies?
An average works only where the variation is genuinely linear across the pour. Where it is not, dividing the area into zones of consistent thickness and summing them is more accurate and avoids a systematic shortfall.
The formula
slab = L x W x depth/12; round = pi x r^2 x height; yards = cubic feet / 27
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How to use this calculator
- Shape. Enter your own figure; the page starts at Slab or rectangle.
- Length (ft) (ft). Enter your own figure; the page starts at 20 ft.
- Width (ft) (ft). Enter your own figure; the page starts at 12 ft.
- Thickness or depth (in) (in). Enter your own figure; the page starts at 4 in.
- Diameter (in) (in). Enter your own figure; the page starts at 12 in.
- Height (ft) (ft). Enter your own figure; the page starts at 8 ft.
The result updates as you type, so you can try a figure, see what it does to cubic yards, 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 |
|---|---|
| Shape | Slab or rectangle |
| Length (ft) | 20 ft |
| Width (ft) | 12 ft |
| Thickness or depth (in) | 4 in |
| Diameter (in) | 12 in |
| Height (ft) | 8 ft |
| How many | 1 |
What comes out
| Result | Value |
|---|---|
| Cubic yards main result | 3.0 |
| Cubic feet | 80.0 |
| Per item | 3.0 |
| 60 lb bags if mixing by hand | 178 |
That is 3.0 cubic yards, 80.0 cubic feet. By hand it would take about 178 sixty-pound bags, which is why anything past half a yard is usually ordered ready-mixed.
How sensitive the answer is
Holding everything else steady and changing only length (ft), this is what happens to cubic yards. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Length (ft) (ft) | Cubic yards |
|---|---|
| 10 ft | 1.5 |
| 15 ft | 2.2 |
| 20 ft example above | 3.0 |
| 30 ft | 4.4 |
| 40 ft | 5.9 |
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
- Shape
- Choose from Slab or rectangle, Round column, Circular footing.
- Length (ft) (ft)
- Measured in ft, starting at 20 ft.
- Width (ft) (ft)
- Measured in ft, starting at 12 ft.
- Thickness or depth (in) (in)
- Measured in in, between 0 and 400, starting at 4 in.
- Diameter (in) (in)
- Measured in in, between 0 and 200, starting at 12 in.
- Height (ft) (ft)
- Measured in ft, starting at 8 ft.
- How many
- A number, between 0 and 999, starting at 1.
What the results tell you
| Result | What it is |
|---|---|
| Cubic yards | The headline result. A decimal figure. |
| Cubic feet | A decimal figure. |
| Per item | A decimal figure. |
| 60 lb bags if mixing by hand | A number. |
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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