Calculating cement coverage by bag size simply involves volumetric calculations. But in order to calculate it correctly, you need all of the pieces of the puzzle to make the correct estimate. There are few things more frustrating than calculating too little cement when you have a huge project you would like to complete by a certain time. The puzzle pieces you will need are: length, width, depth, and the amount of wet cement an 80-pound or 60-pound bag of premixed cement will provide. To illustrate this, our example will involve pouring a new backyard patio.
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Things you need
- Measurements associated with your project
Measure the desired length or your patio (or the project you are working on). For this example, the length of the patio measures 21 feet, 9 inches.
Then measure the desired width of your patio or other project. For this example, the width of the patio measures 16 feet, 3 inches.
Determine the depth of the slab of concrete you would like to pour. For most applications, as with the patio example, a depth of 4 inches is sufficient. But if you need to store heavy items, machinery, vehicles, etc., on this concrete slab, you may want to plan a depth of as much as 6 inches. Again, this example will use a depth of 4 inches.
Because the measurements involve both "foot" and "inch" values, both must be converted into inches to provide a more exact volume estimate. Our measurements converted into inches are as follows: Length = 21 feet, 9 inches = 252 inches + 9 inches = 261 inches Width = 16 feet, 3 inches = 192 inches + 3 inches = 195 inches Depth = 4 inches
Now there is enough information to determine an accurate volume. This calculation is length x width x depth: 261 x 195 x 4 = 203,580 cubic inches Multiplying three values together provides a 3-dimensional value and, in this case, because we are using inches, the value expressed is in cubic inches.
Bagged cement is measured in cubic feet. So the next problem that has to be solved is converting cubic inches into cubic feet. An 80-pound bag of premixed cement = .667 cubic feet, or 2/3 of a cubic foot, of concrete. A 60-pound bag of premixed cement = .5 cubic feet, or 1/2 of a cubic foot, of concrete. To find the volume of one (1) cubic foot of concrete, in inches, simply multiply length x width x depth in inches: 12 x 12 x 12 = 1728 cubic inches
Then, using division and the measurements you have figured out, you can find the number of cubic feet of cement needed to pour this patio: 203,580 / 1728 = 117.8125 cubic feet, or rounded up to the nearest cubic foot, 118 cubic feet.
Now we can find out how many bags of premixed cement we are going to need for this patio. If we are using 80-pound bags, the calculation is: 118 / .667 = 176.911 bags, or 177 bags. Most home-improvement stores will shy away from selling you .911 bags of cement, so round up to the nearest full bag. Now for the 60-pound bags: 118 / .5 = 236 bags Bagged cement can be purchased by the pallet. Pallets of 80-pound cement include 42 bags per pallet. Pallets of 60-pound cement include 54 bags per pallet.
Professional concrete suppliers favour selling concrete by the cubic yard. You can convert cubic feet to cubic yards by using division: One cubic yard = 3 x 3 x 3 = 27 cubic feet 118 cubic feet / 27 = 4.37 cubic yards If you would rather use a cement product that is trucked to your home, call your local concrete supplier.
Tips and warnings
- There is a big difference between cement and concrete. The finished patio is considered a concrete patio. The product used to make the concrete patio is cement. These terms are not interchangeable. The easiest way to remember which is which is: the active ingredient in premixed cement is Portland cement.
- Mix cement products in a well-ventilated area.
- Do not breathe cement and sand dust.
- Use a respirator when mixing cement products.