If you’re like many builders, your favorite activity is putting things together. Figuring out how much you’ll need in materials? Not so much. Unless you’ve mastered this skill, it probably feels like all the calculations and formulas are a difficult mystery. However, it’s much easier than you think to make these calculations, so long as you have the right tools. Let’s talk about what you need to know, and how to do it.
Construction Materials Units
Your first step is to know which measurement units go with which materials. This part is relatively easy, because you can see how each item is purchased. For instance, let’s say you’re a plumbing contractor. Most pipes are sold by material, diameter, and length. But because material and diameter are already specified in the plans, you need to determine the total length necessary. Then, you determine what pipe lengths and joints are needed to total that length. Other materials come in different units of measurement.
In addition to pipes, many other items are measured primarily by length. For instance, all kinds of electrical wiring are measured this way. Sometimes feet are used, and other times it’s in inches. Internationally, you may see metric units as measurements, though manufacturers typically list both unit types. To determine needed length, you’ll add up the lengths of each segment, accounting for joints and any drops or spurs needed.
Your next common measurement is related to area. This means the total flat space that something takes up. For instance, you need the area of a wall to know how much wallboard to purchase, and the area of a floor to purchase the right amount of carpet. Depending on your project, the units will be in square feet or square inches. You’ll determine this by multiplying the length and width of your wall or floor. Then, you’ll subtract the area of any windows, doors, or other large cutout. Then, convert this into the sheets of wallboard based on what’s needed to cover that area.
Finally, many construction material requirements are determined by volume. This is especially true with concrete, asphalt, and certain blocks. It is also the most difficult measurement to make. From a mathematical perspective, you get area by multiplying length, width, and height. While this is enough for something easy, such as a solid concrete block foundation, it’s insufficient if you have anything going through the concrete. This could be rebar, plumbing, or electrical circuits. In this case, you need to determine the volume of these items and subtract it from the total required.
In many ways, this one’s the easiest. A quantity or count is simply the number of something needed to get the job done. So, you’ll use “count” for nails, screws, light fixtures, and appliances. Anything which you can give an exact number by simply adding up each item. While some construction materials of this type end up being sold by weight, you’ll still be able to use that weight to determine the number per sale unit. So, you might buy a pound of screws and know that a certain number of screws make up that pound.
Determining Total Unit Needs
Now that we’ve looked at the types of units which can be counted, let’s talk about how to figure out that total unit requirement. Most of you have heard of this process as the “takeoff.” In a nutshell, takeoffs determine total length, area, and volume requirements. Then, you’ll convert this into whatever sale unit of each material to determine necessary purchases.
Manual Quantity Takeoffs
Manual takeoffs are the tedious task of determining how much of each item is needed by counting, measuring, and calculating. Small projects like a basic deck or simple concrete slab can be taken off easily by an experienced contractor. All others, however, will require some work. Although taking off manually is old school, it’s still done by many estimators and contractors.
Measure Walls, Windows, and Doors
With a scale ruler, you’ll measure around walls. This information is used to determine lengths of certain items, such as pipe and wire, which are sold primarily by length. Then, you’ll measure the height of your walls to determine area. At the same time, you’ll make those same measurements of any windows or doors. Finally, add some length for electrical wire or pipes to account for drops and other necessary diversions from the straight line.
Another thing you’ll need to do is count assemblies. For the sake of this discussion, an assembly could be a light fixture, electrical outlet, or other similar item. These will be added up over time, often using a spreadsheet or similar method. From this you’ll get a total count of items, divided down by type, that need to be bought by the piece.
Determine Framing Needs
Here, you’re measuring and adding up the different pieces of lumber or steel beams that are needed for framing your project. That could be as simple as the beams holding up a deck or as complicated as the joists holding up a high rise building. Over time, you’ll get the total length of these beams. From here, the number of beam pieces is calculated based on the standard sale length of each item. So, a 20 foot beam that’s purchased by five feet would require 4 pieces to complete.
Calculate Roof Area
For roofing, the takeoff needs to determine the area of your roof. Using similar measurements as your walls, you’ll get that total area. Then, that number will be converted into plywood for decking, quantity of housewrap, and shingles or other roofing material.
Other things you’ll need to determine include the volume of concrete needed for a foundation, asphalt for a driveway, and other items. Calculating volumes can be hard if there are a lot of pipes, rebar, or wires to deal with. But it’s relatively easy to figure out how much concrete to buy once you’ve gotten an accurate volume.
Digital Quantity Takeoffs
For digital takeoffs, the process is similar, but the calculations are done primarily by computer. Depending on the program you’re using, there will be a different level of human input required. For instance, some programs will scan the plans and make most of the measurements automatically. In this case, you’ll do little more than tell the program what each symbol means. In other situations, you’ll have to click on each wall segment, symbol, or other plan item to count it. Fortunately, for these the measurements are still made automatically.
With digital takeoffs, there are fewer opportunities for human error. One example of this is the total area of walls which require gypsum wall board, or the shed areas which take plywood. Each item is catalogued as you go along, reducing the chances of losing track. In this way, digital takeoff software is just like a cash register as it adds up your grocery bill.
At the end, a digital takeoff will give a complete report of construction materials needed to complete your project. If you’re doing this along with the overall estimate, you’ll add other items such as equipment rentals before running a total. In addition, many construction takeoff programs include some level of pricing information to help with the estimate.
Figuring Out How to Buy Your Materials
Of course, you can’t exactly buy a 1,000-foot board even if you wanted to. Trees don’t grow that tall, and it isn’t a practical unit for sale either. Other construction materials are also sold in convenient units of sale, and multiple options are often available.
To determine how much of each package to purchase, you’ll first need to look at the standard sales unit for each item. For instance, you might need a total of 36 feet of 2×4 wood. However, these boards typically come in 8 or 12-foot lengths. Your total length need is satisfied by getting 3, 12-foot boards. Get it in 8-foot lengths, and you’ll end up wasting 4 inches, which costs money.
Here’s another example. Concrete is sold different ways, including in the form of cement mix and premixed by truck. Depending on the amount needed and its application, either option might be better. And, in each situation you need to know how many sales units to purchase. Bagged mix will be sold in dry weight, as well.
Finally, you’ll need to account for waste. While we should always try to purchase the closest amount possible to actual needs, this isn’t always doable. Sometimes the problem is availability: if you need 37 feet of 2×4 board, you will need to buy an extra 3 feet because of package sizes. In addition, there are instances of waste which result from the building process itself. One example is cutouts of wall board for windows or doors. You may be able to use some of that waste elsewhere, but sometimes it also gets thrown out. Either way, you may have to buy some material that isn’t used.
However, it’s always best to minimize waste. This costs money, especially if the waste is of a type which can’t be used on another project. So, you might be able to use an extra 3 feet of board for stakes or other purposes. Or, if you have only 6 inches it’s more likely to get tossed. Either way, you need to ensure you don’t throw your profits out with it.
Knowing how much material you need might seem intimidating at first. Fortunately, with a few tools and some know-how, you too can determine what’s required for each project. Of course, if you don’t want to do this manually then you should consider using software or our construction takeoff services.