*Results include the specified waste margin.
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 crucial. Underestimating results in a weak cold joint from a second pour, while overestimating wastes money and requires heavy disposal.
The standard formula for calculating concrete volume is:
Note that the depth is usually measured in inches, so you must divide the inches by 12 to convert to feet before multiplying.
Understanding Cubic Yards
If you are ordering ready-mix concrete from a truck, you will need to know the volume in cubic yards. There are 27 cubic feet in one cubic yard. Our calculator automatically handles this conversion for you.
How Many Bags of Concrete Do I Need?
For smaller DIY projects, you will likely use pre-mixed bags (like Quikrete or Sakrete) from a hardware store. Here are the standard yields:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
To calculate the bags manually: Take your total volume in cubic feet and divide it by the yield of the bag size you are using. Always round up to the nearest whole bag.
Why Include a Waste Margin?
It is standard practice to add a safety margin (usually 5% to 10%) to your order. This accounts for:
- Spillage during the pour.
- Uneven subgrade (dips in the ground) requiring more material.
- Compression of the aggregate.
- Some concrete remaining inside the mixer or wheelbarrow.
Our calculator allows you to input a custom waste percentage to ensure you don't run dry in the middle of your project.
Standard Slab Thicknesses
4 Inches: Standard for walkways, patios, and residential shed floors.
5-6 Inches: Recommended for driveways and garages holding passenger vehicles.
6+ Inches: Heavy-duty use, such as RV pads or commercial areas.