Concrete Calculator
Estimate concrete volume from project dimensions and include a waste allowance. For structural work, always verify the design and product yield.
What is the concrete calculator?
Estimate concrete volume from project dimensions and include a waste allowance. For structural work, always verify the design and product yield.
How to use the calculator
Enter the length, width, and slab thickness. For bagged concrete, use the manufacturer’s yield rather than relying on a generic bag volume when placing an actual order.
How to calculate concrete volume
Convert thickness to feet and multiply length × width × thickness. Divide cubic feet by 27 for cubic yards. For irregular pours, break the shape into rectangles, circles, or other simple sections and add the volumes.
How much extra concrete should I order?
A small allowance helps account for uneven excavation, form dimensions, spillage, and measuring variation. The appropriate allowance depends on the project; ready-mix suppliers may have minimum order and short-load rules.
Ready-mix vs. bagged concrete
Ready-mix is generally used for larger pours, while bags are convenient for small pads and repairs. Bag yield differs by product and should be taken directly from the packaging.
Structural caution
Footings, foundations, retaining structures, beams, columns, and other structural work can require engineering, reinforcement, soil information, and local permits. This calculator is not a structural design tool.
How to improve the accuracy of your estimate
Use the actual product dimensions, package coverage, material density, installed depth, and current local price whenever those values are available. Measure the project in smaller sections when the shape is irregular, and keep fixed costs separate from area-based quantities. For a large purchase, compare the calculator result with the supplier’s own coverage or takeoff before ordering.
Common mistakes to avoid
The most common errors are mixing inches and feet, using nominal dimensions instead of actual product dimensions, forgetting waste or overage, treating volume and weight as interchangeable, and assuming a universal installation depth. Enter the values from your actual product and project specification whenever possible.
Worked planning example
A 20 × 20 ft slab at 4 inches thick is 266.7 cubic feet, or about 9.88 cubic yards before waste. Actual ready-mix ordering should be based on the exact form dimensions and supplier recommendations.
Useful Concrete reference values
| Thickness | Approx. concrete per 100 sq ft |
|---|---|
| 3 in slab | 0.926 cu yd per 100 sq ft |
| 4 in slab | 1.235 cu yd per 100 sq ft |
| 5 in slab | 1.543 cu yd per 100 sq ft |
| 6 in slab | 1.852 cu yd per 100 sq ft |
Reference values are planning aids. Product specifications and local conditions take precedence.
Frequently asked questions
How many cubic yards of concrete do I need?
Calculate the volume in cubic feet and divide by 27. Add a project-appropriate allowance and confirm the final quantity with your ready-mix supplier.
How much concrete is in a 4-inch slab?
A 4-inch slab over 100 sq ft requires about 33.33 cubic feet, or 1.235 cubic yards, before waste.
How many bags of concrete make a yard?
The answer depends on bag weight and product yield. Use the exact yield printed on the product label.
Can I use this calculator for an irregular project?
Yes, but the best approach is to divide the project into measurable sections and combine the results. Irregular edges, curves, openings, and changes in depth can make a single rectangular measurement less accurate.
What should I verify before buying materials?
Verify the product label or manufacturer coverage, actual dimensions, density or package yield, required installation depth, and supplier delivery terms. For structural or regulated work, also verify the project specification and local requirements.