There are a few formulas to learn that will make it easier for you to calculate how many bags of concrete to buy for your project. No matter what size bag you are planning to use, you can calculate how much concrete to buy.
We know that there are 27 cubic feet in a cubic yard – 3 x 3 x 3.
We know that there are 1728 cubic inches in a cubic foot – 12 x 12 x 12.
Now we need to know three more things. How many cubic feet does the bag state that it will make? How many cubic feet do you need for your project? What size bag does your builder’s supply carry?
The brand I buy has a website with all the information you could ever want about the products they make. For an example, let’s use Quikrete Sand Mix. It’s packaged in three different bag sizes. 40# will make .37 cubic feet, the 60# will make .5 cu ft, and the 80# will make .66 cu ft.
If you buy the 80# bag, you can make .66 cubic feet of your project. Since we know there are 1728 cubic inches in a cubic foot, we multiply 1728 x .66 and find that an 80# bag will do 1140 cubic inches of project.
Now, let’s calculate the number of cubic inches we need. For an example, we want to make some concrete flagstones for a small walkway. They will all be the same size – 8 x 12 x 2.5 so that is 240 cubic inches.
One bag will do 1140 cubic inches, so we can do simple division and learn that we can make 4.75 flagstones per bag.
If our project requires only four stones, one bag will do it. If we need 20 stones, we better buy five bags and have some left over for another project. If you have a larger project, you should look into having some sand delivered to your work area and buying a bag of Portland cement. You will save money, but you will have to mix your ingredients.
No matter which way you decide to go, once you have the basic math down pat, you can figure any project and any material.