Quantity take-offs are performed at the start of every estimate. Essentially, they quantify the material needed to complete the work specified for a project.
For every trade there is a standard method of quantifying material in a take-off. Caulking is measured in linear feet, concrete is measured in cubic yards, drywall is measured in sheets, floor tile is measured in square feet, paint is measured by how many square feet it can cover per gallon, etc. We've got an entire section on Earthwork that also touches on measurement of spoils piles, undisturbed earth, end-area method and typical ways to measure earth, or soil, which is considered a material.
Along with material quantification, contractors typically know how much work they can complete in a day, or the production rate. It's important to see the relationship in quantity of materials required to complete the job in relation to labor and equipment costs. This will be discussed more in the Estimating section.
A simple example would entail completing a take-off for CMU blocks needed in a retaining wall. Before the contractor can price the work, he must perform a take-off using plans or engineering documents. Again, in order to complete the take off, a few pieces of information are needed.
Retaining wall: 4'-0" x 75'-0" = 300 sf
CMU Block: 1 block = 8" x 16" = .88 sf
Total block needed for wall:
300 sf / .88 sf = 341 blocks for the wall
Note: we only care about the square footage because the width of the wall is determined by the block. We could perform the take off in terms of volume, but we would get the same answer as both wall and block are 8" wide.
These ideas are for the curious minds out there who beg to see the "big picture."
Now, the masonry contractor will have an idea of how much grout will fill each block. Because this is a more advanced take-off, it could be completed with a measurement of the cavities inside the block. For each block, there are two cavities measuring approximately 7" x 8" x 6". For those of you who'd like to try this on your own, remember to convert cubic inches to cubic feet to cubic yards.
There's also rebar in the block wall, let's say #5 bars, embedded 1'-0" into the concrete below, 4'-0" high, at 16" o.c. with horizontal rebar running through each course. You could convert quantity of bars (by linear feet) to weight of steel and provide a quantity in lbs of steel. Typically, rebar is dowelled out of the slab or footing below, and there is a bar in every other cavity. Corners, end conditions, and locations where two walls intersect will typically have two bars in each cavity. It's quite tricky making sure rebar ends up where it was meant to be, hence the invention of epoxy.
So, once all take-offs related to a portion of work are completed, then an estimate is prepared. Some contractors don't have to do a take-off to know what the work will cost. This is risky business, so that practice is reserved for the old school, seasoned contractors.