Concrete Calculator
Calculate exactly how much concrete volume and pre-mixed bags you need for your next slab, footing, or stairs project.
* Bag estimates include a standard +10% waste margin recommended for all concrete pours.
Concrete Volumes by Shape
The Premise: Buying the wrong amount of concrete can ruin a project. Too little creates a cold joint, while too much wastes money.
The Evidence: The exact mathematical formulas are Length × Width × Depth for a slab. However, this is converted into cubic yards since that is the industry standard unit.
The Conclusion: Utilizing our Concrete Calculator ensures you order exactly the right amount of concrete for any geometric shape, avoiding waste and ensuring structural integrity.
Quick Reference: Concrete Bags & Yards for Common Slabs
Below is an instant estimating reference chart for typical residential concrete slabs (patios, sheds, driveways, sidewalks) poured at standard 4-inch and 6-inch depths. All estimates include a 10% safety margin for subgrade unevenness and spillage.
| Slab Dimensions | Thickness | Cubic Yards | 80 lb Bags | 60 lb Bags |
|---|---|---|---|---|
| 8' × 8' (64 sq ft) | 4 inches | 0.87 yd³ | 40 bags | 53 bags |
| 10' × 10' (100 sq ft) | 4 inches | 1.36 yd³ | 62 bags | 82 bags |
| 10' × 12' (120 sq ft) | 4 inches | 1.63 yd³ | 74 bags | 98 bags |
| 12' × 12' (144 sq ft) | 4 inches | 1.96 yd³ | 88 bags | 118 bags |
| 12' × 16' (192 sq ft) | 4 inches | 2.61 yd³ | 118 bags | 157 bags |
| 10' × 10' (100 sq ft) | 6 inches | 2.04 yd³ | 92 bags | 123 bags |
| 12' × 24' (288 sq ft) | 4 inches | 3.91 yd³ | 176 bags | 235 bags |
| 20' × 20' (400 sq ft) | 4 inches | 5.43 yd³ | 245 bags | 326 bags |
How to Use the Calculator
- Select the shape of your pour: Slab, Column, or Stairs.
- Enter the exact dimensions of your project (Length, Width, and Depth) in your preferred units.
- Click the "Calculate Volume" button.
- Review the cubic yards required, along with the bag count for 40lb, 60lb, and 80lb bags.
Limitations
Please note the following limitations: This calculator assumes a perfect geometric shape. Real-world conditions like uneven subgrades or bowed forms will alter the required volume. Always include at least a 10% waste factor (which this tool calculates automatically).
Data Privacy & Security
Your privacy is important to us. All calculations performed by the Concrete Calculator happen locally within your web browser. We do not store, track, or transmit your project dimensions to any server.
Frequently Asked Questions
Why do I need to add 10% extra?
It is an industry standard to order 10% more concrete than your exact mathematical volume. Forms can bow, the subgrade might be uneven or slightly deeper in spots, and some concrete always spills or remains in the mixer. Running out of concrete mid-pour is a disaster.
Should I use 60lb or 80lb bags?
80lb bags are more economical and mean less mixing, but they are very heavy and can cause back strain if you are mixing a lot of them by hand. 60lb bags are much easier to handle for a single person doing a DIY project.
When should I order a concrete truck instead of bags?
As a general rule of thumb, if your project requires more than 1 cubic yard (about 45 of the 80lb bags), it is highly recommended to order a ready-mix truck. Mixing that many bags by hand or even with a small portable mixer is incredibly labor-intensive.
How do I calculate concrete for a slab?
Multiply length x width x depth (all in feet), then divide by 27 to get cubic yards.
How many bags of concrete make a yard?
It takes forty-five 80lb bags, sixty 60lb bags, or ninety 40lb bags to equal one cubic yard of concrete.
Why does a 4-inch concrete slab actually measure 3.5 inches with 2x4 forms?
Standard dimensional lumber sold as '2x4' actually measures 1.5 inches by 3.5 inches. If you screed concrete flush with 2x4 forms set directly on grade without digging down or elevating the forms, your slab will be 3.5 inches thick rather than a full 4.0 inches. This difference reduces required concrete volume by about 12.5% (saving 0.13 cubic yards per 100 square feet).
How much does a cubic yard of concrete weigh?
A standard cubic yard of poured wet concrete weighs approximately 4,050 pounds (about 2.02 tons). In comparison, buying dry premix bags to yield 1 cubic yard requires 45 eighty-pound bags (3,600 lbs dry weight before water is added). Always check your truck or trailer payload capacity before transporting dry concrete bags.
How to calculate concrete volume for round post holes and sonotubes?
For cylindrical holes (fence posts, deck footings, sonotubes), the formula is Radius (ft) × Radius (ft) × π × Depth (ft). For a standard 12-inch diameter sonotube 4 feet deep (below frost line): Radius = 0.5 ft. Volume = 0.5 × 0.5 × 3.1416 × 4 = 3.14 cubic feet (0.12 cubic yards, or about 6 eighty-pound bags per hole including 10% waste).
What if my pour is a custom shape?
For complex custom shapes, it is best to break the area down into smaller rectangles or cylinders, calculate the volume for each separately, and then add the cubic yards together.
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