Concrete Slab & Bag Calculator
*Estimates include waste margin. Always round up to the nearest whole bag.
How to Calculate Concrete for Your Project
Whether you are pouring a simple backyard patio, a driveway, or footings for a deck, accurate calculation of concrete volume is critical. Ordering too little concrete results in "cold joints" and structural weaknesses, while ordering too much wastes money and creates disposal issues. This calculator helps you determine the exact volume in cubic yards—the standard unit for ordering ready-mix trucks—and estimates the number of premix bags needed for smaller DIY projects.
The Concrete Formula
The basic formula for calculating concrete volume is:
Length (ft) × Width (ft) × Thickness (ft) = Volume (cubic feet)
Since slab thickness is usually measured in inches, it must first be converted to feet by dividing by 12. For example, a 4-inch thick slab is 0.33 feet thick. Once you have the cubic footage, you divide by 27 to get Cubic Yards.
Standard Thickness Guidelines
Choosing the right thickness is essential for the longevity of your slab:
- 4 Inches: Standard for sidewalks, patios, and residential floor slabs.
- 5-6 Inches: Recommended for driveways and areas holding light vehicles.
- 8+ Inches: Required for heavy equipment or industrial foundations.
Understanding Premix Bag Yields
For smaller projects (typically under 1 cubic yard), buying bags of dry concrete mix is more economical than paying the delivery fee for a truck. Yields vary slightly by brand, but generally:
- One 80lb bag yields approximately 0.60 cubic feet of cured concrete.
- One 60lb bag yields approximately 0.45 cubic feet of cured concrete.
Our calculator applies your selected waste margin (we recommend 5-10% to account for uneven subgrades and spillage) to ensure you don't run out mid-pour.