Concrete Slab Calculator
0% (Exact)
5% (Recommended)
10% (High Safety)
Cubic Yards Needed:
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Total Cubic Feet:
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60lb Bags Needed:
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80lb Bags Needed:
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*Calculations include selected waste factor. Bag counts are rounded up to the nearest whole bag.
How to Calculate Concrete Volume
Whether you are pouring a patio, a driveway, or footings for a deck, calculating the correct amount of concrete is crucial. Ordering too little can result in a "cold joint" which weakens the structure, while ordering too much wastes money.
To determine the volume of concrete needed for a slab, you need three measurements: Length, Width, and Thickness.
Pro Tip: Most contractors calculate volume in Cubic Yards, but buy pre-mix bags (like Quikrete or Sakrete) based on weight (60lb or 80lb bags).
The Concrete Formula
The math behind the calculation involves converting all dimensions to feet and then dividing by the cubic footage of a cubic yard (27 cubic feet).
- Step 1: Convert thickness from inches to feet by dividing by 12.
- Step 2: Multiply Length × Width × Thickness (in feet) to get Cubic Feet.
- Step 3: Divide Cubic Feet by 27 to get Cubic Yards.
How Many Bags of Concrete Do I Need?
If you are using pre-mixed bags rather than ordering a truck, use the following yields estimates:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
Example: If you need 10 cubic feet of concrete, you would need roughly 17 bags of 80lb mix (10 / 0.60 = 16.66).
Recommended Thickness for Projects
The thickness of your slab depends heavily on its intended use:
- 4 Inches: Standard for walkways, patios, and residential driveways (passenger cars only).
- 5-6 Inches: Recommended for driveways that hold heavier vehicles (RVs, trucks) or hot tub pads.
- 6+ Inches: Heavy-duty commercial applications.
Why Include a Waste Factor?
It is standard industry practice to add a safety margin, typically 5% to 10%. This accounts for:
- Spillage during the pour.
- Uneven subgrade (ground) depth.
- Some concrete remaining inside the mixer or wheelbarrow.
Running out of concrete mid-pour is a disaster. It is always cheaper to buy two extra bags than to make a second trip to the store while your wet concrete hardens.