*Includes calculated wastage factor.
How to Calculate Concrete for Slabs and Footings
Whether you are pouring a patio, a driveway, or footings for a deck, calculating the correct amount of concrete is crucial. Ordering too little results in a "cold joint" and structural weakness, while ordering too much is a waste of money. Our Concrete Slab Calculator helps you determine exactly how many cubic yards or pre-mix bags you need for your project.
The Concrete Formula
To calculate the volume of concrete required, you must convert all dimensions to a consistent unit (usually feet) and multiply them. The formula for a rectangular slab is:
Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
Since thickness is typically measured in inches, you must divide the inches by 12. For example, a 4-inch slab is 0.33 feet thick.
Converting to Cubic Yards
Concrete is sold by the Cubic Yard. There are 27 cubic feet in one cubic yard. To convert your volume:
- Formula: Cubic Feet ÷ 27 = Cubic Yards
Most ready-mix trucks hold between 8 and 10 cubic yards of concrete. If your project requires less than 1 yard, it is often more economical to mix it yourself using bags.
How Many Bags Do I Need?
If you are using pre-mixed bags (like Quikrete or Sakrete), the yield depends on the bag weight:
- 80lb Bag: Yields approximately 0.60 cubic feet.
- 60lb Bag: Yields approximately 0.45 cubic feet.
- 50lb Bag: Yields approximately 0.375 cubic feet.
This calculator automatically divides your total required cubic footage by these yields to tell you exactly how many bags to buy.
Don't Forget Wastage!
Professional contractors always order more concrete than the exact mathematical volume. Uneven sub-grades, spillage, and form settling can increase the amount needed. We recommend adding 5% to 10% extra to your order to ensure you don't run short during the pour.