The calculation, one unit at a time
The slab is 10 feet long and 10 feet wide. Convert the 4-inch thickness to feet: 4 ÷ 12 = 0.333 foot. Multiply all three finished dimensions:
33.33 ft³ ÷ 27 = 1.23 yd³
A published approximate yield for an 80 lb bag of QUIKRETE Concrete Mix is 0.60 cubic foot. Divide required volume by yield, then round up to a whole bag:
Never round down; a partial calculated bag still requires a full package.
Why the practical answer is often 59
Forms and excavation are rarely perfect mathematical boxes. A modest allowance can cover small depth variation, subgrade irregularity, spillage, and concrete left in tools or the mixer. With a 5% allowance:
- 33.33 ft³ × 1.05 = 35.00 ft³.
- 35.00 ft³ ÷ 0.60 ft³ per bag = 58.33 bags.
- Round up to 59 bags.
That order is 4,720 pounds of dry mix before water, plus the work of moving and mixing 59 separate bags. The quantity is large enough that a local ready-mix quote deserves comparison, even if delivery minimums or access ultimately make bags the better choice.
It helps with quantity variation. It does not determine suitable thickness, reinforcement, base preparation, joints, drainage, or load capacity.
What if the store has another bag size?
Use the yield printed for the exact mix. For the same 35.00 cubic feet including 5% allowance, common published approximate yields give:
| Bag size | Approx. yield | Bags for this example |
|---|---|---|
| 40 lb | 0.30 ft³ | 117 bags |
| 60 lb | 0.45 ft³ | 78 bags |
| 80 lb | 0.60 ft³ | 59 bags |
Bag count alone does not choose the best package. Compare price per supplied cubic foot, lifting weight, mixer capacity, labor, storage, and the batch-to-batch consistency you can maintain.
Confirm these details before buying
- Actual finished thickness. A 5-inch slab uses 25% more concrete than a 4-inch slab of the same length and width.
- Exact product yield. High-strength, fast-setting, and specialty mixes may not match the common figures above.
- Forms and base. Measure after the base is compacted and forms are set where practical.
- Batch plan. Confirm the mixer, crew, water source, placement time, and finishing sequence can handle the volume.
- Local design requirements. Slab use, soil, frost, drainage, reinforcement, and code can change the specification.
Source and scope
Approximate bag yields come from the QUIKRETE Concrete Mix product data sheet: 0.30 ft³ for 40 lb, 0.45 ft³ for 60 lb, and 0.60 ft³ for 80 lb. Verify the current data sheet for the product you buy. This is a material estimate, not engineering advice.