Concrete calculator: how many bags of concrete do I need?
Enter the slab size and thickness. The calculator works out the volume in cubic yards (or m³), adds waste, and converts to 80 lb or 60 lb bags at the yield printed on the bag, or to ready-mix.
Price this job as an estimate in Quotesprig (free)
Common sizes, worked out
4 in thick slab, 10% waste, 80 lb bags at 0.6 cu ft per bag. Every number below comes from the same calculator code as the tool above.
| Job | Slab volume | With waste | 80 lb bags | Labour |
|---|---|---|---|---|
| 10 × 10 ft slab | 1.23 cu yd | 1.36 cu yd | 62 bags | 2.5 hr |
| 12 × 12 ft slab | 1.78 cu yd | 1.96 cu yd | 88 bags | 3.6 hr |
| 10 × 20 ft slab | 2.47 cu yd | 2.72 cu yd | 123 bags | 5 hr |
| 20 × 20 ft slab | 4.94 cu yd | 5.43 cu yd | 245 bags | 10 hr |
| 10 × 10 ft slab, 6 in thick | 1.85 cu yd | 2.04 cu yd | 92 bags | 2.5 hr |
Assumptions you can change
Starting values: waste allowance 10 %, yield per 80 lb bag 0.6 cu ft, yield per 60 lb bag 0.45 cu ft, your production rate (place + finish) 40 sq ft/hr. They are typical starting points, not rules: use the numbers on your product labels and your own production rate.
FAQ
How many bags of concrete for a 10x10 slab?
For a 10 × 10 ft slab (4 in thick slab, 10% waste, 80 lb bags at 0.6 cu ft per bag): slab volume 1.23 cu yd; with waste 1.36 cu yd; 80 lb bags 62 bags. Change any input above for your own job.
How much does an 80 lb bag of concrete make?
About 0.6 cubic feet is the figure the calculator starts with, which is what many 80 lb bags list; use the yield printed on your bag.
When should I order ready-mix instead of bags?
Once a job needs more than roughly a cubic yard, mixing dozens of bags by hand gets slow; switch "Supply as" to Ready-mix to see the volume to order, rounded up to the nearest quarter yard.
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