*Results include the waste margin specified.
How to Calculate Concrete for a Slab
Planning a concrete project requires precise volume calculations to avoid running out of material mid-pour or overspending on excess. Whether you are pouring a patio, a driveway, or a shed foundation, the formula relies on the dimensions of your slab.
The basic formula for concrete volume in cubic yards is:
(Length (ft) × Width (ft) × Depth (ft)) / 27
Since slab thickness is usually measured in inches, you must first divide the inches by 12 to get feet. For example, a 4-inch slab is 0.33 feet thick.
Standard Slab Thicknesses
- 4 Inches: The standard thickness for residential sidewalks, patios, and garage floors for passenger cars.
- 5-6 Inches: Recommended for driveways that hold heavier vehicles (like RVs or trucks) or heavy-use garage floors.
- 6+ Inches: Heavy-duty commercial applications or structural foundations.
Why Include a Waste Margin?
It is critical to order slightly more concrete than your exact mathematical calculation. This accounts for:
- Uneven subgrade (dips in the ground).
- Spillage during the pour.
- Settlement of forms.
Most contractors recommend adding 5% to 10% extra concrete to your order. Our calculator includes a default 5% safety margin, which you can adjust based on your site conditions.
Premix Bags vs. Ready-Mix Truck
If your project requires less than 1 cubic yard of concrete, buying premixed bags (60lb or 80lb) from a hardware store is usually cost-effective. For projects requiring more than 2 cubic yards, ordering a ready-mix truck is typically more efficient and provides a more consistent mix.