Knowing the bag yield does two jobs: it tells you how many bags to buy, and it tells you the point where bags stop making sense and a ready mix truck takes over. Both come from one fixed number per bag, so this is worth committing to memory if you pour concrete more than once.
What each bag yields
A bag of concrete mix yields a fixed volume of wet, placed concrete. The water you add does not change the finished volume much, so these figures are reliable across brands.
| Bag size | Yield | Bags per cubic yard | Bags per cubic foot |
|---|---|---|---|
| 80 lb | 0.60 cu ft | about 45 | 1.67 |
| 60 lb | 0.45 cu ft | about 60 | 2.22 |
| 50 lb | 0.375 cu ft | about 72 | 2.67 |
| 40 lb | 0.30 cu ft | about 90 | 3.33 |
Where the numbers come from
A cubic yard is 27 cubic feet. Divide 27 by the yield of a bag and you get the bags per yard. For 80 pound bags, 27 divided by 0.60 is 45. For 60 pound bags, 27 divided by 0.45 is 60. The same logic gives bags per cubic foot if you divide 1 by the yield.
Worked example
You need 0.5 cubic yards for a set of footings. That is 27 times 0.5, or 13.5 cubic feet. At 0.60 cubic feet per 80 pound bag, that is 13.5 divided by 0.60, about 23 bags before waste. Add a couple for spillage and round up to 25. If you only had 60 pound bags, the same job would take about 30 bags.
Choosing a bag size
Larger 80 pound bags mean fewer bags to open and mix, but each one is heavy to lift and carry. Smaller 40 or 60 pound bags are easier on your back and easier to handle in a small mixer, at the cost of more bags overall. For more than a handful of bags, the larger size usually wins on effort.
When to switch to ready mix
Mixing 45 bags to make a single cubic yard is a long, physically hard day, and batches mixed minutes apart can form cold joints where they meet. Once you approach a cubic yard, ready mix delivered by the truck is usually cheaper once you value your time, and it gives a single continuous, consistent pour.
Metric bags
Metric concrete bags are commonly 20 or 25 kilograms. A 25 kilogram bag yields roughly 0.0117 cubic metres and a 20 kilogram bag about 0.0094, so it takes around 85 of the 25 kilogram bags to make a cubic metre, or about 106 of the 20 kilogram bags.