Concrete Slab Calculator
Cubic Yards Needed:
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Cubic Feet Needed:
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60lb Premix Bags:
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80lb Premix Bags:
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Note: Calculations include your selected waste margin to account for spillage and uneven subgrades.
How to Calculate Concrete for a Slab
Calculating the correct amount of concrete is crucial for any construction project, whether you are pouring a patio, a driveway, or a simple shed foundation. Ordering too little concrete can result in structural weaknesses called "cold joints," while ordering too much is a waste of money.
The formula for calculating concrete volume is relatively simple:
- Step 1: Convert all dimensions to feet. Since thickness is usually measured in inches, divide the inches by 12. For example, a 4-inch slab is 0.33 feet thick.
- Step 2: Multiply Length × Width × Thickness to get Cubic Feet.
- Step 3: Divide Cubic Feet by 27 to convert to Cubic Yards, which is the standard unit for ordering ready-mix concrete trucks.
Pre-Mix Bags vs. Ready-Mix Truck
If your project requires less than 2 cubic yards of concrete, it is often more economical to mix it yourself using pre-mix bags (like Quikrete or Sakrete). Here is a general rule of thumb:
- 60lb Bag: Yields approximately 0.45 cubic feet of concrete.
- 80lb Bag: Yields approximately 0.60 cubic feet of concrete.
For large projects exceeding 2 cubic yards, ordering a ready-mix truck is usually preferred to ensure consistency and save manual labor.
Example Calculation
Imagine you are pouring a 10ft x 10ft patio that is 4 inches thick.
1. Volume in cubic feet: 10 × 10 × (4/12) = 33.33 cubic feet.
2. Volume in cubic yards: 33.33 / 27 = 1.23 cubic yards.
3. Bags needed (80lb): 33.33 / 0.60 = 56 bags (plus waste margin).
Always add a 5-10% safety margin (waste factor) to account for spillage, slab settling, or uneven ground depth.