Concrete Slab & Premix Bag Calculator
How to Calculate Concrete Slab Materials
Calculating the amount of concrete needed for a slab requires determining the total volume of the area and converting that volume into the specific units sold by retailers, such as cubic yards for ready-mix trucks or number of bags for smaller DIY projects. The standard formula involves multiplying length by width by depth, ensuring all units are consistent.
The Concrete Volume Formula
To determine the cubic footage of your slab, use the following calculation:
- Step 1: Measure Length and Width in feet.
- Step 2: Measure Thickness in inches and divide by 12 to convert to feet.
- Step 3: Multiply: Length × Width × Thickness (in feet) = Cubic Feet.
- Step 4: To get Cubic Yards (for truck delivery), divide Cubic Feet by 27.
Understanding Premix Bag Yields
If you are mixing the concrete yourself using premix bags from a hardware store, you need to know the yield of each bag size. This calculator uses standard industry yields:
- 80lb Bag: Yields approximately 0.60 cubic feet of cured concrete.
- 60lb Bag: Yields approximately 0.45 cubic feet of cured concrete.
Note: It is industry standard to add a 5% to 10% safety margin (waste factor) to account for spillage, uneven sub-grade depths, or texture variations. Our calculator includes a customizable waste margin field to ensure you don't run short in the middle of a pour.
Thickness Guidelines
The thickness of your slab depends on its intended use:
- 4 Inches: Standard for residential sidewalks, patios, and garage floors for light vehicles.
- 5-6 Inches: Recommended for driveways holding heavier vehicles (SUVs, trucks) or areas with poor soil support.
- 6+ Inches: Heavy-duty industrial floors or areas supporting heavy machinery.
Cost Estimation Factors
This tool provides a material cost estimate based on the price per unit you enter. Keep in mind that this covers the concrete only. A complete project budget should also include costs for lumber (forms), gravel (sub-base), rebar or wire mesh (reinforcement), and finishing tools.