A BTU (British Thermal Unit) is a measure of heat energy. One of these units signifies how much energy it will take to heat one lb. (473ml.) of water one degree Fahrenheit hotter than it is already (for example, heating 0.454kg. of water form 18.3 to 18.9 degrees Celsius). Household room heaters come in different BTU sizes, with the required BTU for a room being determined by multiplying the room's volume by its quality of wall/floor/ceiling insulation.
Measure both the length, width and height of the room that needs to be heated (measure in feet and inches).
Multiply these three measurements together. This calculation will give you the room's cubic volume.
Rate the room's insulation in the walls, ceiling and floor. If the insulation is very good, give it a No. 2 rating. If the insulation is medium quality, give it a No. 3 rating. If the insulation is poor, give it a No. 4 rating.
Multiply the room's calculated volume by its insulation rating. This number will give you the number of BTU's required to heat the room.
It is best to error on the side of caution when choosing the correct size heater for a room. For example, if the calculation for the room comes to 15,000 BTU's, and the heater you are purchasing comes in 15,000 and 20,000 BTU , it is best to choose the 20,000 BTU heater. This is especially so in colder or wetter climates.