How to use
- Choose slab or post-hole mode.
- Enter the project dimensions and bag yield.
- Review total volume, waste allowance, and bag count before purchasing.
Input guide
- Depth is entered in inches for typical slab and hole dimensions.
- Bag yield is cubic feet per bag; use the label on the bag when available.
- Waste covers spillage, uneven subgrade, and small measurement differences.
Common mistakes
- Using slab thickness in inches as if it were feet.
- Forgetting to subtract a post inside each hole.
- Ordering exactly the calculated volume with no margin.
Assumptions and limitations
- No rebar, footing, load, permit, or structural slab design is included.
- Post-hole estimates assume straight cylindrical holes.
- Bag yields are typical defaults and can vary by product.
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Formula
- Slab volume = length x width x depth in feet.
- Adjusted volume = volume x (1 + waste percent / 100).
- Bags needed = adjusted volume / selected bag yield, rounded up.
- For post holes, cylinder volume is reduced by the optional post volume.
Example
A 10 ft x 10 ft slab at 4 in deep is 33.33 cu ft. With 10% waste and 80 lb bags yielding 0.60 cu ft, buy 62 bags.
FAQ
Does this replace ready-mix ordering advice?
No. It is for planning small bagged-concrete purchases and shows cubic yards for comparison only.
Should I round concrete bags up?
Yes. The calculator rounds up because partial bags usually cannot be purchased or used cleanly.