How to Calculate Concrete for a Slab
Whether you are pouring a patio, a driveway, or a shed foundation, calculating the correct amount of concrete is critical. Ordering too little results in a weak "cold joint" when you have to pour a second batch later, while ordering too much wastes money.
To determine the volume of concrete needed, you must calculate the cubic footage of the slab and then convert that to cubic yards (for ready-mix trucks) or the number of premix bags (for smaller DIY projects).
The Concrete Formula
The math behind the calculation follows a standard volume formula:
Volume = Length × Width × Thickness
However, because the length and width are usually measured in feet and the thickness in inches, you must convert the thickness to feet first. The formula used in this calculator is:
- Step 1: Convert Thickness to Feet: Thickness (in) ÷ 12
- Step 2: Calculate Cubic Feet: Length (ft) × Width (ft) × Thickness (ft)
- Step 3: Calculate Cubic Yards: Cubic Feet ÷ 27
60lb vs. 80lb Premix Bags
For smaller projects (typically under 1 cubic yard), buying bags of premix concrete from a hardware store is often more economical than ordering a truck. This calculator estimates the number of bags you need based on typical yields:
- 80lb Bag: Yields approximately 0.60 cubic feet of cured concrete.
- 60lb Bag: Yields approximately 0.45 cubic feet of cured concrete.
Always round up to the nearest whole bag. It is much better to have one bag left over than to run out halfway through your pour.
Why Include a Safety Margin?
Excavations are rarely perfectly flat. If your ground is slightly uneven, or if your forms bow out slightly under the weight of the wet concrete, you will need more material than the perfect mathematical volume. Industry standard suggests adding 5% to 10% extra material (the "waste factor") to account for spillage, uneven subgrades, and form settling.
Common Concrete Thicknesses
- 4 Inches: Standard for residential sidewalks, patios, and garage floors (for passenger cars).
- 5-6 Inches: Recommended for heavy-duty driveways or floors supporting heavy machinery.