Concrete Calculator
Calculate cubic yards or meters of concrete needed for slabs, footings, and columns with optional waste factor.
Concrete Calculator tool
Example
Input: Slab 12 ft x 12 ft, 0.5 ft (6 in) thick, 10% waste included
Result: 2.93 cubic yards (79.20 cu ft, 2.24 cu m); 2.67 cubic yards without waste; 132 80lb bags
12 x 12 x 0.5 = 72 cu ft, plus 10% = 79.2 cu ft; 79.2 / 27 = 2.93 cu yd, and 79.2 / 0.6 cu ft per 80 lb bag = 132 bags.
Concrete for Common 4-Inch Slabs
| Slab size (ft) | Cubic yards | With 10% waste | 80 lb bags |
|---|---|---|---|
| 4 x 4 | 0.20 | 0.22 | 10 |
| 8 x 8 | 0.79 | 0.87 | 40 |
| 10 x 10 | 1.23 | 1.36 | 62 |
| 10 x 12 | 1.48 | 1.63 | 74 |
| 12 x 12 | 1.78 | 1.96 | 89 |
| 10 x 20 | 2.47 | 2.72 | 123 |
| 12 x 24 | 3.56 | 3.91 | 177 |
| 20 x 20 | 4.94 | 5.43 | 245 |
| 24 x 24 | 7.11 | 7.82 | 353 |
Bags include the 10% waste allowance, at 0.6 cubic feet per 80 lb bag.
- Choose imperial (feet) or metric (meters) and select the shape of your pour - slab, footing, or column.
- Enter the length, width, and depth for a slab or footing, or the diameter, height, and number of columns.
- Leave the 10% waste option on to account for spillage and form irregularities, or turn it off to see the exact volume.
- Review the total volume in cubic yards or cubic meters, plus the estimated number of 80 lb pre-mixed bags for smaller jobs.
- Copy the results or adjust dimensions to compare different project sizes and material requirements.
How Much Concrete Do You Need?
To find how much concrete you need, multiply length x width x thickness in feet, then divide by 27 to get cubic yards. A 12 x 12 ft slab that is 6 inches (0.5 ft) thick holds 72 cubic feet, or 2.67 cubic yards. With 10% extra for waste, order 2.93 cubic yards.
Pick the shape you are pouring. A slab or footing is a box: length x width x depth. A column, such as a round post hole or a tube form, uses pi x radius squared x height, times the number of columns. Enter every size in the same unit, feet or meters. Thickness catches people out. Four inches is 0.33 ft, not 4, and 6 inches is 0.5 ft.
The 10% waste allowance is on by default. It covers spills, uneven ground and forms that bow a little. Running out partway through a pour leaves a weak joint where fresh concrete meets concrete that has started to set. That costs far more than a little extra mix. Turn the option off to see the exact volume. The result also shows the volume without waste, so you can compare.
For small jobs, the tool counts 80 lb bags of premixed concrete. Each bag makes about 0.6 cubic feet, so one cubic yard takes 45 bags. That is a lot of mixing by hand. For larger pours, a ready-mix delivery is usually easier, and suppliers sell by the cubic yard or cubic meter. For setting fence posts, pair this with the Fence Calculator. To convert volumes, use the Volume Converter.
Formula / How It Works
Frequently Asked Questions
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