20×12 slab, 4" thick
- Cubic feet = 80
- Yards ≈ 2.96
- Bags ≈ 133
construction calculator
Concrete Volume Calculator
Estimate cubic yards, cubic feet, and bag counts for slabs, footings, and post holes.
Results
- Cubic yards
- 2.96
- Cubic feet
- 80.00
- 80 lb bags
- 133.33
How to use this calculator
- Enter length and width in feet, thickness in inches.
- We convert thickness to feet, compute cubic feet and cubic yards.
- Bag estimate uses ~45 bags per cubic yard for 80 lb mix.
Inputs explained
- Length/Width
- Dimensions of the slab/footing in feet.
- Thickness
- Depth in inches; common slabs are 4–6 inches.
How it works
We convert depth from inches to feet, compute cubic feet (L × W × D), divide by 27 for yards, and multiply by 45 bags/yard for 80 lb mix.
Formula
Cubic feet = L × W × (depth ÷ 12)\nCubic yards = cubic feet ÷ 27
When to use it
- Estimating ready-mix yardage for slabs or footings.
- Sizing 80 lb bag counts for small DIY projects.
- Comparing cost between bagged mix and delivered concrete.
Tips & cautions
- Add 5–10% for waste, spillage, and uneven subgrade.
- Slabs thicker than 4–6 inches or with beams/recesses need separate volume calculations.
- If using 60 lb bags, multiply the 80 lb bag count by ~1.33.
- Assumes rectangular footprint; irregular shapes need segmentation.
- Bag-per-yard factor is approximate; check manufacturer yield for your mix.
- Does not include rebar, reinforcement, or sub-base materials.
Worked examples
Deep dive
This concrete volume calculator converts slab/footing dimensions into cubic yards and 80 lb bag counts. Enter length, width, and thickness to size your order before pouring.
Use it for slabs, footings, or small pads. Add waste, check mix yield, and adjust for 60 lb bags if needed.
FAQs
- Should I add extra?
- Yes. Order 5–10% more to cover waste and uneven ground.
- How many bags per yard?
- Roughly 45 bags of 80 lb mix per cubic yard. Check your product’s yield to confirm.
- Can I use this for piers or sonotubes?
- This calculator is for rectangular volumes. For round piers, calculate volume separately (πr²h) and convert to yards/bags.
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