The total footprint of a 14×20 garage is 280 square feet. Keep in mind that a footprint of 280 square feet does not equate to 280 square feet of usable interior space. To get a calculation of usable interior space, you need to remove at least 5″ from all sides. This allows for 3.5″ for the wall studs, and up to 1.5″ of sheathing and siding on the outside of the building. Thus, the usable interior space for a 14×20 garage should be calculated by multiplying 13 feet 7 inches by 19 feet 7 inches, resulting in approximately 266 square feet of usable floor space.