Concrete Slab & Footing Calculator
0% (Exact)
5% (Recommended)
10% (Complex shapes)
Project Totals
Total Volume:
0 Cubic Yards
Cubic Feet:
0 ft³
Pre-Mix Bag Estimate:
80lb Bags Needed:
0
60lb Bags Needed:
0
*Estimates include the selected waste margin.
How to Calculate Concrete Volume
Whether you are pouring a new patio, a driveway extension, or footings for a deck, getting the concrete volume right is the first step to a successful project. Ordering too little concrete results in expensive delays and cold joints, while ordering too much is a waste of money.
To calculate the concrete needed for a slab, you need to determine the volume in cubic yards. The formula is:
Volume = Length (ft) × Width (ft) × Thickness (ft)
Since most tape measures are in inches, ensure you convert your thickness into feet (divide inches by 12) before multiplying. For example, a 4-inch slab is 0.33 feet thick.
Understanding Pre-Mix Bags vs. Ready-Mix Trucks
Once you have your total volume, you need to decide how to buy your materials:
- Small Projects (Under 1 Cubic Yard): It is usually more cost-effective to buy pre-mix bags (like Quikrete or Sakrete) from a home improvement store. Standard sizes are 60lb and 80lb bags.
- Large Projects (Over 1 Cubic Yard): For volumes greater than 1 yard, ordering a ready-mix truck is often easier on your back and wallet. One cubic yard of concrete weighs approximately 4,000 lbs—far too much to mix by hand efficiently.
How Many Bags Do I Need?
The yield of concrete bags varies slightly by brand, but standard yields are:
- 80lb Bag: Yields approximately 0.60 cubic feet. You need roughly 45 bags to make 1 cubic yard.
- 60lb Bag: Yields approximately 0.45 cubic feet. You need roughly 60 bags to make 1 cubic yard.
Don't Forget the Waste Margin
The ground is rarely perfectly flat. Excavation inconsistencies, spillage during pouring, and form bowing can increase the amount of concrete required. We highly recommend adding a 5% to 10% safety margin to your calculation to ensure you don't run short.