Material calculator
Concrete Bag Calculator
Estimate concrete volume and bag count for slabs, patios, walkways, or post holes.
These results are planning estimates only. Verify product labels, site conditions, local requirements, and professional guidance before buying or building.
Calculator inputs
Adjust the typical defaults below. Results update automatically.
Results
Review quantities, warnings, and the shopping list before buying.
Concrete volume
33.33cu ft
Concrete volume
1.235cu yd
Waste allowance
10%
Bags to buy
62bags
80 lb bag
Material list
- 62 bags 80 lb bag0.6 cu ft yield each
- 10 % Concrete waste allowanceIncluded in bag count
Breakdown
| item | calculation | value | unit |
|---|---|---|---|
| Slab volume | 10 ft x 10 ft x 0.333 ft | 33.33 | cu ft |
| Adjusted volume | Volume x 1.1 waste factor | 36.67 | cu ft |
| Bag count | 36.67 / 0.6, rounded up | 62 | bags |
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.
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.