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Concrete Calculator: Volume and Number of Bags
Estimate how much concrete a slab, pad or round column needs. You get the volume in cubic yards, cubic meters and cubic feet, plus the count of bagged mix, with a waste allowance you control.
How the volume is worked out
Concrete is ordered by volume, and volume is a simple product. A rectangular slab is length × width × thickness. A round column or pier is a cylinder: π × (diameter ÷ 2)2 × height. The calculator converts everything to cubic meters internally, multiplies by the number of identical pieces, adds your allowance, and then shows the result in the units suppliers use.
Thickness and diameter are entered in inches or centimeters because that is how they are normally specified, while length, width and height are in feet or meters. One cubic yard is 27 cubic feet, or about 0.7646 cubic meters.
Turning volume into bags
Bagged mix is sold by weight, and the volume it produces after water is added is called its yield. Divide the total volume in liters by the yield of one bag and round up. Small jobs under roughly a cubic yard are usually done with bags; larger pours are normally ordered as ready-mix delivered by truck, where you quote the volume in cubic yards or cubic meters.
Brands differ slightly, so the yield is an input. Copy the number from the label of the bag you plan to buy rather than relying on the defaults.
Common ways to under-order
- Forgetting the base. A sub-base that dips by half an inch across a patio quietly adds several percent to the volume.
- Measuring the nominal thickness. Forms set for a 4 inch slab on uneven ground often finish thicker at one end.
- Using no allowance. Pumping losses, wet edges and a last wheelbarrow that runs short are all normal. The default 10% is a starting point, not a rule.
- Ordering by the exact figure. Truck minimums and rounding mean you may pay for a little more than you calculate.
What this estimate does not cover
This tool estimates quantity only. It does not choose a mix strength, reinforcement, joint spacing, curing method or footing depth, and none of those can be decided from a calculator. Structural elements such as footings, foundations and suspended slabs should follow local building rules and the advice of a qualified engineer or contractor. Weather, drainage and soil conditions also affect what is appropriate for a given site.
Worked example
A 12 ft by 10 ft patio slab poured 4 inches thick, with a 10% allowance and 80 lb bags that yield about 17 liters each.
Inputs
| Units | Imperial (feet and inches) |
|---|---|
| Shape | Slab, pad or footing (rectangle) |
| Length (feet or meters) | 12 |
| Width (feet or meters) | 10 |
| Thickness (inches or centimeters) | 4 |
| Waste and spillage allowance | 10 % |
| Yield of one bag | 17 liters |
Result
About 74 bags of 17.0 liters each, including the allowance.
| Volume before allowance | 1.133 m³ (1.481 yd³) |
|---|---|
| Volume with allowance | 1.246 m³ |
| In cubic feet | 44.0 ft³ |
| In liters | 1,246 L |
| Bags needed (rounded up) | 74 |
Estimating note: Quantities and ratings here are estimates for planning and learning. Products, local codes and site conditions differ, so check manufacturer data and local regulations, and use a qualified professional for electrical, structural or safety-critical work.
Frequently asked questions
How many bags of concrete do I need for a 10 by 10 slab 4 inches thick?
The slab holds about 33.3 cubic feet. With 80 lb bags yielding 0.6 cubic feet each that is roughly 56 bags before any waste allowance, which is why large slabs are usually ordered ready-mixed.
How do I convert cubic feet to cubic yards?
Divide by 27, because a cubic yard is 3 ft by 3 ft by 3 ft. A volume of 54 cubic feet is therefore 2 cubic yards.
How much extra concrete should I order?
Many people add five to ten percent for spillage and uneven ground. Choose the figure that suits the accuracy of your site measurements and ask your supplier about minimum loads.
Does this work for footings and steps?
Yes for any rectangular footing, and you can add several simple pieces as separate runs. Stepped shapes are best split into rectangles and added together.
Why is the bag count a whole number?
You cannot buy part of a bag, so the count is rounded up. Buying one bag extra is usually cheaper than stopping a pour short.
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References and sources
- American Concrete Institute: publications and guidance on concrete practice. Link
- Bag yield figures are published on manufacturers' product labels and data sheets.