Formula
Cement is one part in four of a 1:3 mix. The dry volume is 1.33 times the wet mortar. A 70-pound bag is 70/94 cubic feet at the bulk density of a 94-pound portland sack:
bags = mortar × 1.33 × 0.25 / (70 / 94)
The result is the exact bag count. Buy the next whole bag. Packaged concrete mix is a different product and stays on the bag page.
Assumptions
- The mix is 1 part cement to 3 parts sand by volume.
- Dry ingredients occupy 33% more volume than the wet mortar.
- The 70-pound bag has the same bulk density as a 94-pound sack.
Worked example
Eight cubic feet of mortar need 3.57 bags of 70-pound masonry cement.
Input definitions
- Dry allowance
- Extra volume because sand has voids before it is mixed.
- 70-pound bag
- A common masonry-cement weight, not the 94-pound portland sack and not an 80-pound concrete mix.
Limitations
The yield printed on the bag replaces this bulk-density assumption. Lime, pigment, and air content are not separate ingredients here.
Questions
Where does 94 come from?
A US sack of portland cement is 94 pounds and about 1 cubic foot. The 70-pound bag is counted as 70/94 of that cubic foot.
Is this the concrete bag count?
No. That page uses 0.60 or 0.45 cubic feet of packaged concrete per bag.