Bagged mix and ready-mix are the same material bought two ways. The volume you need does not change. What changes is the unit: bags are counted, ready-mix is ordered by the cubic yard. The concrete calculator and the concrete slab calculator show both for the same pour.
How many bags make a cubic yard
A cubic yard is 27 cubic feet. Quikrete and Sakrete publish the same yields for standard concrete mix, so the count is simple division.
| Bag | Makes | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Whichever size you buy, one cubic yard is 3,600 lb of dry mix to carry, open and mix: 45 bags × 80 lb.
A worked comparison
A patio slab 12 ft × 10 ft × 4 in is 40 cu ft, which is 1.48 cu yd. With 10% added it is 44 cu ft, or 1.63 cu yd.
- In bags: 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb. That is 5,920 lb of mix.
- As ready-mix: 1.63 cu yd, rounded up to whatever step the supplier sells in.
To compare cost, multiply the bag count by the shelf price, then ask a supplier for a delivered price on the cubic yards.
What a ready-mix order should allow for
NRMCA, the ready-mix industry association, says to order 4% to 10% more than the volume worked out from the plan dimensions. Its reasons are waste, over-excavation and other contingencies. Two details from the same guidance show how little it takes to come up short:
- Hardened concrete in place can be about 2% less in volume than it was when freshly mixed.
- A slab meant to be 4 in thick that ends up 1/8 in thicker needs 3% more concrete.
This is one association’s guidance in two of its documents. We found no figure from Quikrete or Sakrete for bagged mix. The calculators start at 10% and let you change it.
Where the cut-off is
None of the sources we rely on names a volume at which a truck becomes the better choice, so we do not give one. It depends on prices and on things only a local supplier can tell you:
- the price per cubic yard and the smallest load they will deliver
- any charge for a small load or for delivery
- how long the truck can wait while you place the concrete
Ask for those, then set the total beside the price of the bags. Count the labor as well: every cubic yard in bags is 3,600 lb mixed by you.
One limit on bagged mix
Quikrete recommends its concrete mix for pours 2 in thick or more. A thinner layer calls for a different product, and its yield will not match the figures above.