Turn slab volume into a bag count by dividing required volume by the yield printed for the specific bag and rounding up. A 10 ft by 6 ft slab at 4 inches is 20 ft³; at 0.60 ft³ per 80 lb bag, 20 ÷ 0.60 = 33.3, so 34 bags are needed. Bag weight alone does not state yield.
Worked example with numbers
For a 10 ft × 6 ft × 4 in slab, first convert 4 inches to 0.333 ft. Volume is 10 × 6 × 0.333 = about 20.0 ft³. An 80 lb product labelled 0.60 ft³ per bag gives 20.0 ÷ 0.60 = 33.3 bags, which rounds to 34 whole bags. A 60 lb product with a 0.45 ft³ yield would need 20.0 ÷ 0.45 = 44.4, or 45 bags.
How to make the calculation
Measure form length, width, and actual depth after base preparation; use the smallest confirmed depth only if that is truly the planned uniform thickness. Use the concrete-volume calculator for metres and centimetres or equivalent dimensions, then convert its result to the same cubic unit as the bag label. Divide by printed yield, not by pounds or kilograms.
Check the units and assumptions
Concrete volume is length × width × thickness. Bag yield is the hardened or placed volume a stated product produces after mixing, expressed in ft³, L, or m³. The number of bags is required volume ÷ yield per bag, rounded up because a partial bag cannot be reliably supplied. Keep mix type and bag size with the calculation.
Common mistakes
Do not use a cement bag yield for a concrete mix, or assume all 80 lb bags yield the same volume. Do not leave thickness in inches while length and width are in feet. Do not round 33.3 down to 33. Do not use a volume estimate as a reason to add water beyond the product instructions, since extra water can change the mix properties.
When this result does not apply
This article converts a measured volume to a labelled package yield. It does not design slab thickness, reinforcement, joints, sub-base, drainage, curing, structural loading, or the correct concrete class. Large pours may be more appropriately supplied by ready-mix, and local requirements or an engineer can control the specification.
Source for the external fact
QUIKRETE lists approximate yields of 0.60 ft³ for an 80 lb bag, 0.45 ft³ for a 60 lb bag, and 0.30 ft³ for a 40 lb bag of its Concrete Mix in its published technical data. The named product values illustrate why the bag label is the source of truth for bag count.
Related calculators
Calculate slab volume with the concrete calculator, estimate a footing separately in the foundation-concrete tool, and use the volume converter before dividing when the plan and bag label use different units.