Concrete Slab & Bag Calculator
Calculate exactly how much concrete you need in cubic yards and premix bags.
Calculation Results
*Includes selected safety margin. Bags are rounded up to the nearest whole bag.
How to Calculate Concrete for Slabs
Determining the correct amount of concrete for a project is critical to avoid running out mid-pour or overspending on materials. Whether you are pouring a patio, a driveway, or a simple walkway, the math relies on three key dimensions: length, width, and thickness.
The Concrete Formula
The standard formula for calculating concrete volume is:
(Length × Width × Thickness) ÷ 27 = Cubic Yards
Note: Since length and width are usually measured in feet and thickness in inches, you must first convert the thickness to feet (divide inches by 12) before multiplying. Finally, divide the total cubic feet by 27 to get cubic yards, which is the standard unit for ordering ready-mix trucks.
Thickness Guide for DIY Projects
- 4 Inches: Standard for residential sidewalks, patios, and garage floors (for passenger cars).
- 5-6 Inches: Recommended for driveways holding heavier vehicles (SUVs, trucks) or light industrial use.
- 8+ Inches: Heavy-duty commercial aprons or areas with heavy machinery.
Should I Buy Bags or Order a Truck?
If your project requires less than 1 cubic yard of concrete, it is often more cost-effective and convenient to use premix bags (60lb or 80lb) from a hardware store. For projects requiring 2 cubic yards or more, ordering a ready-mix truck is usually cheaper and ensures a more consistent pour.
Why Add a Safety Margin?
Professional contractors always add a "waste factor" or safety margin, typically 5% to 10%. This accounts for:
- Spillage during transport or pouring.
- Uneven subgrade (dips in the ground) that require more concrete to fill.
- Settling of the formwork.
Our calculator defaults to a 10% safety margin to ensure you have enough material to finish the job correctly.