Concrete Slab & Footing Calculator
Calculate exactly how much concrete, cement bags, and pre-mix you need for your project.
Calculation Results
How to Calculate Concrete for Slabs
Calculating the correct amount of concrete is crucial for any construction project, whether you are pouring a patio, a driveway, or a shed foundation. Ordering too little results in expensive delays and "cold joints," while ordering too much is a waste of money.
The standard formula for calculating concrete volume is:
- Length (ft) × Width (ft) × Thickness (ft) = Cubic Feet
- Cubic Feet ÷ 27 = Cubic Yards
Since concrete is typically sold by the Cubic Yard (for truck deliveries) or by the bag (for smaller DIY projects), converting your dimensions correctly is step one. Note that thickness is usually measured in inches, so you must divide the inch value by 12 to get feet.
Understanding Concrete Bags (60lb vs 80lb)
For smaller projects, you will likely buy pre-mixed bags of concrete. The yield of these bags is standard across most brands:
- 80lb Bag: Yields approximately 0.60 cubic feet of cured concrete.
- 60lb Bag: Yields approximately 0.45 cubic feet of cured concrete.
Our calculator automatically divides your total required volume by these yield figures to tell you exactly how many bags to buy.
Why Add a Waste Margin?
We highly recommend adding a "waste factor" or margin of safety—typically 5% to 10%. This accounts for:
- Spillage during mixing or pouring.
- Uneven subgrade (the ground is rarely perfectly flat).
- Slight variations in formwork dimensions.
It is always cheaper to buy two extra bags now than to drive back to the store in the middle of a pour.