Concrete Slab Volume Calculator
Calculate cubic yards and bag counts for your project
Total Volume:
0.00 Cubic Yards
Volume in Cubic Feet:
0.00 cu ft
80lb Bags Required:
0 Bags
60lb Bags Required:
0 Bags
How to Calculate Concrete for a Slab
Whether you are pouring a patio, a driveway, or a shed base, knowing exactly how much concrete to order is critical. Concrete is typically measured in Cubic Yards. Ordering too little leads to cold joints and structural weaknesses, while ordering too much is a waste of money.
The Slab Formula
To calculate the volume of a rectangular slab manually, you use the following steps:
- Length × Width: Calculate the surface area in square feet.
- Convert Thickness: Divide the thickness in inches by 12 to get feet (e.g., 4″ = 0.33 ft).
- Cubic Feet: Multiply Area × Thickness (in feet).
- Cubic Yards: Divide the total cubic feet by 27 (since there are 27 cubic feet in one cubic yard).
Why Include a Waste Margin?
In real-world construction, we always add a 10% waste factor. This accounts for variations in the subgrade depth, spillage, or slight spreading of the forms. If your calculation says you need exactly 1.0 yards, ordering 1.1 yards ensures you don't run short halfway through the pour.
Bagged Concrete Yields
If you are mixing bags yourself for smaller projects, keep these standard yields in mind:
| Bag Size | Yield per Bag | Bags per Cubic Yard |
|---|---|---|
| 80 lb Bag | 0.60 cubic feet | 45 Bags |
| 60 lb Bag | 0.45 cubic feet | 60 Bags |
| 40 lb Bag | 0.30 cubic feet | 90 Bags |
Pro Tips for a Successful Pour
- Check Level: Ensure your forms are perfectly level or sloped 1/8 inch per foot for drainage.
- Compaction: Compact the sub-base (gravel/dirt) before calculating, as loose soil will sink and increase the concrete volume required.
- Reinforcement: Don't forget to account for rebar or mesh placement, though these generally don't displace enough concrete to change your volume calculation significantly.