Lumber is usually sold either in bulk by the board foot or in 16 foot long boards. To bid on some jobs you may need to estimate board feed. The formula to calculate board feet is length in inches multiplied by width in inches multiplied by height in inches divided by 144. Math phobic carpenters and contractors might make an estimate of the board feet they will need then add 20 percent to that. A more detailed approach, considering exactly what you are going to make and how you are going to make it, is more accurate.
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Draw your project with every board in that project on a piece of paper. Identify the width, thickness and length of every board. Make separate lists for every plank of different widths and every stud, strip or timber you will need. Think of planks of different widths and thicknesses, studs, strips and timbers as separate categories.
Add up all the lengths in all those categories. Divide the total length in feet in each category by 16 and round up. You might need, for example, three 16 foot lengths of 1 x 8 plank, two of 1 x 6 and one length of 4 x 4 timber.
Calculate the cuts you will need to make in all those 16 foot lengths of wood.
Add the end cuts for wood that has been slightly damaged in a lumber yard and allow for the kerfs, or the width of the cuts, you will make. For example, a 16 foot long plank will never yield two, true eight foot planks because some of the wood, the kerf, will be destroyed in cutting.
Draw representations of all the 16 foot lengths of each plank, stud, strip or timber with representations of the cuts you will make. Add the number of boards at your lumber yard that you expect to be cupped (warped horizontally) or bowed (warped vertically) from poor storage or handling.