4 Easy Ways to Calculate Linear Feet - wikiHow... mean the same thing.) Many common construction materials (like lumber and steel, for instance) are measured in feet and sold by the foot. The width of the lumber may be a factor in its price (wider boards cost more), but for most lumber that you buy, you just need to know the linear feet. For some projects, like building a backyard deck, you will need to be able to convert square footage into linear feet of building material. In this case, the width of the material is part of the calculation.