To begin any construction work, it is important to identify the amount of concrete that you need. You want to be as accurate as possible in buying the right amount of materials. You can find several concrete calculators online that say the same figures, but in an actual world, you may want to know the estimated number of bags that you need per yard or area that you will work on. Knowing the numbers or the exact mathematical computation per pound is not as practical as knowing the amount needed per bag or packaging. 

So how do you calculate the amount of concrete needed for any home or commercial projects? 

How many 80 pound bags of concrete does it take to make a yard?

Short answer:

45 bags. 

An 80-pound bag of pre-mixed cement and concrete is 0.6 cubic feet per the bag

Since there are 27 cubic feet in a cubic yard, so the equation is 27 / 0.6 = 45

Long answer:

If you are amazed by a bit of mathematical computation, then read on. The first thing that you need to do is to determine the volume of the space that you want to fill. This is not as complicated at all. Filling out a concrete or chocolate ice cream cone is no different. Determining the volume is quite easy. Due to the amount of concrete needed for most construction jobs, it is best to measure the volume in cubic feet. But if your project is massive, then it is recommended to calculate the volume in cubic yards instead of cubic feet. 

To simplify things, whenever you hear the word cube, think of an ice cube. The ice cube has three sides – about the sides that touch the ice tray in your freezer. The simplest way to get the volume of an ice cube is the multiple these three sides. If you have a perfect ice cube, the width, length, and depth will be the same. For example, it will be 2 feet wide, 2 feet long, and 2 feet deep. Therefore, when these figures are multiplied, you will get a volume of 8 cubic feet. 

Refer to this formula: 

Volume = width x length x height (depth) 

Meanwhile, concrete slabs’ dimensions are not equal. They are not perfect cubes. That is why the computation is a bit different. If you have a slab that is both 2 feet wide and long, the depth or height unlikely to be 2 feet. It might be 4 inches in depth for an average concrete slab. Hence, follow this simple formula: 2’ x 2’ x 4”. But you cannot multiply feet by inches. You have to compute everything in the same units. So, after multiplying everything, simply divide it by 12. You’ll get the answer 1.3 cubic feet. 

Refer to this formula: 

2 feet x 2 feet x 4 inches 

2 x 2 x 4 = 16 / 12 = 1.3 cubic feet 

For filling a round hole in the street to install a post or mailbox, the equation for the volume is different from the cube or rectangle.

The formula for the volume of a round hole is: 

Pi r2 x depth 

3.14 x radius x radius x depth 

The width of the radius is the half portion of the round hole. But if you are not working on a big construction project, this will be easily calculated in terms of inches. 

If your round hole is 36 inches deep and 10 inches wide, the equation would be:

 3.14 x 5 x5 x36 /1728 = 1.6 cubic feet 

In case you forget the divisor which is 1728, just remember this formula: 12 x 12 x12 or simply the figure of cubic inches in a cubic foot. 

Now that we’re done calculating the volume, the next step is to calculate the number of concrete bags you will need to fill the round hole. When you walk into your local hardware store, you will usually see on the packaging or product label, how many cubic feet the bag can fill. This refers to the yield. The yield on an 80 pounds bag is about 6/10 or 0.6 cubic feet. Given the example mentioned above, the equation would be: 

1.6 / 0.6 – 2.7 bags 

The yield is almost always constant. You can use 0.06 yield on any calculation.