Concrete Calculator: Estimate Slab Volume and Bags
Free concrete calculator to estimate cubic feet, cubic yards and 80 lb ready-mix bags for a rectangular slab.
How to estimate concrete for a slab
Multiply slab length by width in feet, then multiply by thickness converted from inches to feet. Divide the volume by the yield listed on your concrete bag, allowing for extra material.
Example estimate
A 10 by 8 foot slab that is 4 inches thick has about 26.67 cubic feet of concrete, or 0.99 cubic yards. With a 10% allowance and 0.6 cubic feet per 80 lb bag, the estimate is 49 bags.
Before you buy materials
Bag yield varies by brand and mixing conditions. For load-bearing or structural slabs, consult a qualified professional. All results are planning estimates, not professional construction advice.
Calculate slab volume
For a rectangular slab, multiply length × width × thickness in feet. Convert thickness in inches to feet by dividing by 12. The result is cubic feet; divide by 27 for cubic yards.
Worked bag estimate
A 10 × 8 ft slab at 4 inches thick needs about 26.67 cubic feet before extra material. Adding 10% gives 29.33 cubic feet. At an assumed 0.6 cubic feet per 80 lb bag, round up to 49 bags. Check the actual yield printed on your chosen mix.
Common questions
Does every 80 lb bag have the same yield?
No. Use the manufacturer's stated yield rather than relying on the example value.
Does this estimate include a gravel base?
No. Base material, forms, reinforcement, and finishing supplies are separate.
Can I use this for a structural slab?
The estimate is only for volume. Structural or load-bearing projects need qualified design and applicable permits.