The total footprint of a 12×30 garage is 360 square feet. Keep in mind that a footprint of 360 square feet does not equate to 360 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 12×30 garage should be calculated by multiplying 11 feet 7 inches by 29 feet 7 inches, resulting in approximately 346.5 square feet of usable floor space.