A bill of materials is a compiled list that specifies the materials used to build a building or residence. When a contractor or home builder prepares to build, they first use a mathematical process to calculate the amount of lumber, doors, shingles, windows, etc., will be needed to complete the project.
The process starts with the floor plan where studs and plates are calculated using the linear dimension of the walls. This process is used to calculate both the interior and exterior walls.
Afterwards, exterior sheathing and interior sheetrock is calculated by multiplying the height of the walls by the length which results in a square footage figure which is divided into the amount of sheets used. This process is also used for the floor decking and ceiling sheetrock. Then trim for the baseboard and openings is calculated using the length of the walls.
The foundation or basement is generally figured in the same way. In other words the wall area is calculated to square inches and then divided by the amount of square inches in a masonry block. Most masonry block manufacturers have the formula to tell how much mortar is needed to put the blocks together. The footings are calculated by the amount of cubic yards are contained in them.
Next the doors, windows, counters & cabinets, plumbing fixtures, are counted along with the electrical outlets, light fixtures, switches, wire lengths, receptacle boxes, and all other remaining items. These items are summed up by the number of times they appear on the plan. The wiring is calculated by length measurements from one fixture to the next.
There are specific formulas or methods used to finally end up with a list that can be carried to a building supply for pricing. Sometimes the building supply company will complete this list for you in order to insure your business.