The size of room that a particular space heater is sufficient to heat is calculated by dividing the heater's output in British thermal units by 34.1. The result is the approximate maximum size of room in square feet that the space heater is capable of heating.
The formula for calculating the maximum appropriate room size for a given space heater only produces an estimate of the heating power of the appliance, as reported by retail website Overstock.com. Rooms with many windows, poor insulation or high ceilings increase the amount of thermal power required to adequately heat the room, as can cement or tile floors. Conversely, a smaller space heater is often adequate to heat a larger room than this estimate indicates if the room in question is well-insulated or occupied by several people at most times.
The heating technology used by a particular space heater is also important in determining the appropriate heater for a given space, according to website Power to Change. Space heaters that work by heating objects with infrared radiation are more efficient for spot heating of smaller spaces or particular regions in a room, while convection space heaters are more efficient for heating an entire room.