Concrete Slab & Volume Calculator
How to Calculate Concrete for Slabs
Planning a patio, driveway, or shed foundation requires precise measurements to ensure you order enough material without overspending. This Concrete Slab Calculator helps you determine exactly how many cubic yards or premix bags you need based on the dimensions of your project.
Understanding the Formula
Concrete volume is calculated by multiplying length by width by height (thickness). However, since lumber and measuring tapes use different units, the calculation requires conversion:
- Length & Width: Measured in feet.
- Thickness: Measured in inches (must be converted to feet by dividing by 12).
- The Math:
(Length × Width × (Thickness ÷ 12)) = Cubic Feet.
To get Cubic Yards (the standard ordering unit for ready-mix trucks), divide the Cubic Feet by 27.
How Many Bags of Concrete Do I Need?
If you are mixing the concrete yourself using pre-mixed bags from a hardware store, the conversion depends on the bag size:
- 80lb Bags: You typically need 45 bags to make 1 cubic yard.
- 60lb Bags: You typically need 60 bags to make 1 cubic yard.
Note: These figures include a standard yield. Our calculator automatically applies the waste factor you select to ensure you don't run out mid-pour.
Recommended Thickness for Projects
Not sure how thick your slab should be? Here are industry standards:
- 4 Inches: Standard for patios, sidewalks, and shed floors.
- 5-6 Inches: Recommended for driveways carrying passenger vehicles.
- 6+ Inches: Heavy-duty slabs for RVs or heavy machinery.
Why Include a Waste Factor?
Professional contractors always order slightly more concrete than the exact mathematical volume. Spillage, uneven subgrades (the dirt below the slab), and minor form bowing can increase the volume needed. We recommend a 5% to 10% safety margin to prevent the disaster of running short while the concrete is hardening.