Formula
The page rounds up to a whole bag:
bags = ceil(cubic yards × 27 / bag yield)
Those yields are the figures printed on typical US packaged concrete. The concrete page is the one that starts from length, width, and thickness.
Assumptions
- The yardage is the volume you intend to place.
- The bag yield matches the brand you buy.
- A partial bag is bought whole.
Worked example
Two cubic yards of the 80-pound mix need 90 bags.
Input definitions
- Bag yield
- The wet volume one bag is supposed to make, in cubic feet.
- Cubic yard
- 27 cubic feet. Forty-five of the 80-pound bags fill one yard at the 0.60 yield.
Limitations
Hot weather, too much water, and a yield printed differently on the bag all change the count.
Questions
Why 45 bags per yard?
27 divided by 0.60 is 45. The 60-pound bag yields 0.45 cubic feet, which is 60 bags per yard.
What if I only know the slab size?
Use the slab page for cubic yards, or the concrete page if you want bags directly from the box.