Concrete Slab Calculator
0% (Exact)
5% (Recommended)
10% (Safe)
15% (Complex Forms)
Please enter valid positive numbers for dimensions.
Estimated Materials Needed
Total Volume (Cubic Yards):
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Total Volume (Cubic Feet):
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60lb Pre-Mix Bags:
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80lb Pre-Mix Bags:
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*Calculations include selected waste factor. Bag counts are rounded up to the nearest whole bag.
How to Calculate Concrete for Slabs
Whether you are pouring a patio, a driveway, or a shed foundation, accurately estimating the amount of concrete needed is crucial to the success of your project. Ordering too little results in expensive delays and "cold joints," while ordering too much is a waste of money.
This calculator determines the volume of your slab by using the standard industrial formula: Length × Width × Depth. However, because concrete is sold by the Cubic Yard but measured in Feet and Inches, unit conversion is the most common source of error for DIYers.
The Concrete Formula Explained
To calculate the required concrete manually, follow these steps:
- Convert Thickness to Feet: Since your length and width are in feet, your depth (inches) must be converted. Divide inches by 12. For example, a 4-inch slab is 0.33 feet thick.
- Calculate Cubic Feet: Multiply Length × Width × Thickness (in feet).
- Convert to Cubic Yards: Divide the total Cubic Feet by 27 (since there are 27 cubic feet in one cubic yard).
Pro Tip: Always include a "Waste Factor." Ground unevenness, spillage during the pour, and material remaining in the mixer or truck chute usually account for 5% to 10% of volume loss. Never order the exact mathematical amount.
Truck vs. Bags: Which should you choose?
Once you have your estimate, you need to decide between ordering a Ready-Mix truck or buying pre-mix bags (like Quikrete or Sakrete).
- Pre-Mix Bags (60lb or 80lb): Best for small projects under 1 cubic yard. If your calculator shows you need 45 bags, mixing that by hand or with a small rental mixer is labor-intensive and time-sensitive.
- Ready-Mix Truck: generally recommended for projects over 1 to 1.5 cubic yards. It guarantees a consistent mix and saves back-breaking labor.
Standard Slab Thicknesses
- 4 Inches: Standard for walkways, patios, and residential shed floors.
- 5-6 Inches: Recommended for driveways and areas accommodating light vehicles.
- 6+ Inches: Heavy-duty applications, such as RV pads or foundations for heavy machinery.