Square feet do not "contain" linear feet, but they are calculated using linear feet as units of measurement. Square feet are two-dimensional measurements, whereas linear feet are one dimensional. Because they do not measure the same things, it is not possible to convert from one to another.
Linear feet are used to measure the perimeter of locations like rooms. This is useful for knowing the amount of trim that the walls in a room need or the amount of fence needed to contain a pasture. Square feet measure the total area of a floor, wall or other surface. This is useful for knowing how much paint, carpet or tile would cover those spaces. A single square foot is defined as the area of a square with sides that are a linear foot in length. When people talk of "converting" linear feet to square feet, what they really mean is calculating the area of a place based on its linear foot measurements.