Discover how to correctly determine the required units of measure for different stages of construction planning, such as purchasing materials and performing labor. This article delves into the mathematics behind figuring out the right measurements for materials like concrete, using examples like footing and slab on grade.
Key Insights
- The units of measure for purchasing and performing labor for the same material can differ. For example, while the cubic feet measurements are used in the initial calculations for a concrete footing, the final estimate for both labor and material purchase is based on cubic yards.
- Knowing the correct units of measure before embarking on a construction project is vital. To determine the unit of measure for material purchase, you can check how the material is being sold by supply houses. For labor costs, you can consult with subcontractors.
- The method of calculating measurements can vary based on the type of construction work. For instance, the labor for slab on grade construction is usually calculated per square foot, while the material needed is determined in cubic yards.
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Various items require different units of measure based on how the material is going to be purchased or how it’s installed. You will find as we go through this that the same object may require different units of measure for purchasing versus performing the labor. Here we're looking at concrete.
This is a footing, and we're going to identify how the calculations are used for a cubic yard. So you have a length, you have a width, and you have a depth. That’s required to calculate your total cubic yardage.
So let's look at this diagram. We're going to identify that length times width times depth gives you volume, and in this particular case, it's going to give you cubic feet—but the estimate is going to price things out based on the cubic yard. Let's work with the length of 4 feet times the width of 4 feet times the depth of 3 feet.
It gives us a total of 48 cubic feet. Now, since we're looking for cubic yards—which is how labor and material are priced out—we would then take that 48 cubic feet and divide it by 27, since 27 cubic feet equal one cubic yard. That gives you a total of 1.78 cubic yards per footing, but we're not done there.
That's for a single footing. Throughout the rest of the project, you’ll notice there are 12 of them, resulting in a total of 21.33 cubic yards when you multiply by 12 footings altogether for the project. This is very basic, very simple math.
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It's geometry that you need to understand, but note that according to that image, you must have your length, your width, and your height. But what happens after that? We don't want to work with cubic feet because all of those measurements are in feet and inches. We do want to convert it to yards. Now you know that you have to purchase a little over 21 cubic yards of concrete and also account for the labor to perform that.
You might be wondering, how do you know what unit of measure to use for a concrete footing? By looking at some of the supply houses and inquiring how the material is being sold, that will tell you exactly what the unit of measure is or needs to be on your own takeoff. As far as the labor goes—and if you're uncertain about how it’s priced out—check with your subcontractors. They'll tell you right away that they price their concrete footings by the cubic yard.
You have to know the correct units of measure prior to your takeoff. It’s a requirement so that you know how to measure them. Here we have an example of slab on grade.
It also has a length, a width, and a depth, so we can calculate cubic yards of material—but the labor itself is priced out per square foot, and typically it’s a crew that does the installation. So, you might have multiple people involved with providing even a single square foot of slab on grade. Slab on grade is typically going to be anywhere from 4 inches, perhaps, to a total of 8 or more inches.
So, in this particular case, you can see that you have a length of 24 feet on the slab on grade, then you have a width of 12 feet. So, the slab on grade itself is 288 square feet times 0.667—that’s the decimal equivalent to 8 inches. We’ll discuss those decimal equivalents in more detail shortly.
That equals a total of 192 cubic feet or 7.11 cubic yards of concrete. So, your material is going to be 7.11 cubic yards, but your labor is going to be 288 square feet of slab on grade.
